I will begin by assuming that everyone knows what rhyme is; that will save on what is a much harder term to explain than anyone who hasn't tried it would guess.  In English rhyme is important, not just in itself, but because it largely structures the stanza form as well.  (In ancient Greek and Roman poetry, it was the meter that gave form to the stanza, but since in English nearly everything is in imabic meter, meter alone is not adequate for that function.)
    The most basic unit of rhyme, of course, it that of two rhyming lines, a couplet.  Most rhyming couplets are in either tetrameter or pentameter.  English metrical verse does not seem to take well to the really long line, and shorter than tetrameter moves too quickly and brings the rhymes too close together for most purposes.  There are, of course, exceptions.  See Robert Frost's "A Considerable Speck," for example.  Rather than violate Frost's copyright, however, I will qote some lines of iambic trimeter from my own "The Price of Barbed Wire":

                                             It doesn't make good sense
                                             To have too strong a fence;
                                              Consider the expense,
                                              To build, and then maintain.
                                              Besides, what does one gain?--
                                              No one could ever say
                                              That prisons pay their way,
                                              Or hold in untterly,
                                              For some will still get free,
                                              And now and then mere chance
                                              Creates the circumstance, . . .
    I don't think this is too bad, but we wouldn't want a lot of poetry that sounds like this.  Iambic tetrameter couplets, however, have their own problem.  The four stress line is the most natural rhythm in English, and coupled with as obvious a rhyme scheme as the couplet with rhymes so close together,  the form is a temptation to jingle.  There is a term for sub-literary jingling; it is called doggerel, and most doggerel is in iambic tetrameter couplets.  A great deal of humorous and satiric verse is as well, though the couplet beginning "In Flanders fields . . ." quoted on the page about metrics is in this form.  So is Yeat's "Why Should Not Old Men be Mad," and most of Swift's poetry, as in these lines from "Verses on the Death of Dr. Swift":

                                    As Rochefoucauld his maxims drew
                                    From nature, I believe 'em true:
                                    They argue no corrupted mind
                                    In him; the fault is in mankind.
                                       This maxim more than all the rest
                                    Is thought too base for human breast.
                                    "In all distresses of our friends
                                    We first consult our private ends,
                                    While Nature, kindly bent to ease us,
                                    Points out some circumstance to please us."
    A very different effect is produced by the iambic pentameter couplet.  This form became a standard part of English poetry with Chaucer's Canterbury Tales.  Chaucer's couplets are what are called open couplets.  There is no direct connection between the rhyme scheme and the sentence structure.  A new sentence can begin at any point in a couplet and continue to any point in the next one.  The closed couplet, which has a more extensive history of use than the open couplet, on the other hand, is a closed unit.  Each couplet is either a complete sentence, or at least a fairly self-contained clause.  Twenty syllables is long enough for a complete statement, especially in the compressed language of poetry, and the form is ideal for extensive revisions.  Material can easily be added or eliminated without disturbing the neighboring couplets, and couplets can easily be pulled out of context for revision, even for changing the rhyme word.  It is also a good form for quotable sayings and witty thrusts.  It is not surprising that this form was predominant in the period 1660-1800, the age of wit and reason.  Its two best know practicioners were John Dryden, the dominant poet of the second half of the seventeenth century, and Alexander Pope, the dominant poet of the first half of the eighteenth century, though it was also used by most other poets of the era, even American poets, though with very limited success.  A poet of a colonial and frontier society found it difficult to match the literary elegance and polish of the home country.  As a result, American poets of the eighteenth century produced less of value than those of the seventeenth, even though the seventeenth hadn't raised the bar very high.
    One of the most pervasive features of the closed, or heroic couplet is a feature called "balance and antithesis."  The second line of the couplet is given a strong break (caesura) after the fourth or sixth syllable, and each part of the line has a noun and a verb antithical to the noun and/or verb in the other.  Here is an example by Pope from The Rape of the Lock:

                                  The hungry judges soon the sentence sign,
                                   And wretches hang that jurymen may dine.

This is more an effect one would assciate with poetry of wit and intellect than with anything heroic.  The term heroic couplet comes from the fact that just after the theaters were reopened in 1660, there was a fashion for what was called Heroic Plays, operatic nonmusicals filled with grand passions and set in exotic locals.  These were written in closed couplets, and so gave the form its name. 

    Nobody writes heroic couplets anymore.  The form was pretty much killed by Alexander Pope who brought it to a perfection that could not be matched.  As a young poet, Lord Byron used it in English Bards and  Scotch Reviewers, but never tried it again.  Probably the two best and most original poems in the form after Pope were Samuel Johnson's Vanity of Human Wishes, which is so much slower and more weighty than Pope's verse that it has a feel of originality, and Oliver Goldsmith's Deserted Village, which makes the form more relaxed and homy than it had ever seemed before.  Pope is young, and not at his fullest power as a poet in The Rape of the Lock, but this passage from that poem is a good representative of the form:
                              Close by those meads forever crowned with flowers,
                               Where Thames with pride surveys his rising towers,
                               There stands a structure of majestic frame,
                                Which from the neighboring Hampton takes its name.
                                Here Britain's statesmen oft the fall foredoom
                                Of foreign tyrants and of nymphs at home.

Note the strong balance and antithesis in the last line.  Here is another passage, this one from Sir John Denham's Cooper's Hill, which also takes the Thames river as its setting.  It is also a good statement of what the writers of this age hoped to do with their favorite verse form:

                                O could I flow like thee, and make thy stream
                                My great example, as it is my theme!
                                Though deep, yet clear, though gentle, yet not dull,
                                Strong without rage, without o'erflowing full.
    The next obvious form is the triplet, three rhyming lines.  Triple rhymes are not as easy in English as in some languages, and the triplet seems to have no special resonance in the human soul, and so are uncommon.  A more interesting take on the three line stanza is terza rima.  In this form, lines one and three rhyme.  The second line does not rhyme with either, but is picked up in the next stanza, rhyming with line one and three of that stanza, and so on in an interlocking pattern--obviously much harder to revise than the closed couplet.  Ending such a structure would seem to be a problem, since the middle line is always calling for another stanza to provide the rhyme, but in fact, it can be ended very neatly--simply drop the middle line in the last stanza and close with a couplet.  Doing this draws the rhymes closer together, and gives the final stanza a snap that rounds off the whole canto or verse paragraph neatly.  It is not a common form in English, probably because it demands a series of triple rhymes.  The most famous example of it is Italian, Dante's Divine Comedy.  By far the most famous English example is Shelley's "Ode to the West Wind."  Here is the first section:
O wild West Wind, thou breath of Autumn's being,
Thou, from whose unseen presence the leaves dead
Are driven, like ghosts from an enchanter fleeing,

Yellow, and black, and pale, and hectic red,
Pestilence-stricken multitudes, O Thou,
Who chariotest to their dark wintery bed

The winged seeds, where they lie cold and low,
Each like a corpse within its grave until
Thine azure sister of the Spring shall blow

Her clarion o'er the dreaming earth, and fill
(Driving sweet buds like flocks to feed in air)
With living hues and odours plain and hill:

Wild Spirit which art moving everywhere;
Destroyer and Preserver; hear, O hear!
    Apparently Shelley did not find the rigors of the form too daunting, for he uses it again in the much longer The Triumph of Life.  The whole Divine Comedy, however, was too daunting for the American poet, John Ciardi, for in his very fine translation, he rhymes lines one and three, and simply gives up on rhyming the middle line of each stanza with anything.  That still leaves him the compact and emphatic couplet at the end of each canto.
    I am tempted to stop here with three line units, but considering the popularity of haiku in the second half of the 20th century and since, I need to say a little of it.
     Haiku is a Japanese form that uses neither rhyme nor meter.  It consists of three lines, the first of five syllables, the second of seven, and the third of five.  Whether these numbers are really suitable for an English equivalent is an open question, since Japanese could be either wordier or more compact than English, but this will probably remain the norm for English as well, since the form needs consistency.  Without meter or rhyme or a refrain, the strongest pattern of repitition in haiku is the way its length so exactly mirrors that of every other haiku.  The first line of a haiku deals with something static, seemingly timeless, the second with something abrupt or in motion, the third with the poet's awareness of the two elements coming together.  This form became very popular with American poets in the second half of the twentieth century, and with school writing classes.  Many editors of poetry magazines after a time became quite hostile to the form, since it is so easy to write badly and in quantity.  The fact is that most Americans probably should not write haiku at all.  It works well for a Japanese sensibility and for those Westerners who do have the proper kind of mindset for it, but those are a distinct minority.
    With the four line stanza the possibilities become quite numerous.  Possible rhyme patterns are AABB, ABBA, ABAB, ABCB, and AABA.  If one factors in different line lengths, or alternating line lengths the number becomes very large indeed.  There are some patterns that are not possible--ABAC, for instance.  You can't start with rhyme and then lose it at the end.  Also, if only one line is left unrhymed, it can only be the third.  I will leave it to reader to figure out why. 
    The AABB pattern, of course, is merely a pair of couplets, but couplets that have been bound together as a four line unit.  ABAB is relatively common, but the most common of all four line patters is ABCB with lines one and three being iambic tetrameter and lines two and four being iambic trimeter.  If the four beat line is a very natural rhythm to English, so is the four-three pattern; it is a ratio found very widely in both nature and art, and seems also to be in harmony with some inner rhythm.  This pattern is called ballad meter because it is the most common form for folk ballads--that is, traditional poem/songs that tell a story.  It was used, in fact, probably because it was easy--it could be written with some proficiency by ear without any real understanding of prosody.
    Many poems fall into four line stanzas of one sort or another.  Those in iambic pentameter can be slow and dignified.  Short lines can be quick to the point of frantic.  I will restrict my discussion to ballad meter.  The folk ballad is very old, and some of the poems have a long history of popularity. "Barbara Allen", for instance, is known in both England and America, thouugh it has a different tune in one place than in the other, and fewer than half the lines are identical.  The same is true of several other poems.  By the fifteen hundreds the folk ballad had generated a commercial offshoot, street ballads.  These were topical poems, probably in many cases produced commercially, and often dealing with topical events in a stylized and often lurid manner.  Many of the true folk ballads deal with love, betrayal, jealousy, and murder, and so it does not require a long step to the tabloid-like street ballads.  As a result the term "ballad" had very negative connotations for several centuries.  And even the genuine folk ballad, with its unpolished language had little appeal to sophisticated readership.  Samuel Johnson expresses his attitude toward the tradition with this little parody of the form:

                                                   I put my hat upon my head,
                                                   And walked into the strand.
                                                   And there I met another man,
                                                   Whose hat was in his hand.

Notice that Johnson gets the rhyme scheme and meter exactly right for the typical ballad, though in fact some ballads are in other forms.
Here is a representative stanza from the ballad, "Barbara Allen":

                                                    "Oh, Mother, Mother, make my bed,
                                                    Make it long and narrow.
                                                    My lover died for me today;
                                                    I'll die for him tomorrow.

Here is the opening of the American ballad, "Frankie and Johnnie":

                                                     Frankie and Johnnie were lovers;
                                                     Lordy, how they could love,
                                                     Swore to be  true to each other,
                                                     True as the stars above.

Both of these ballads are not merely read, but have some currency as songs.  The first served as theme song for Bradley Kinkaid's radio show in the 1930's; the second has been performed by Jimmy Rodgers, Johnny Cash, Elvis Presley, and many others.
    Both of these ballads deal with that ballad favorite, love and betrayal, But there are other topics.  "Edward" is the story of a woman who leads her son on to murder his father, and "Sir Patrick Spens" with a disaster at sea:

                                               O Lang, lang may the ladies stand,
                                               Wi their gold combs in their hair,
                                               Waiting for their own dear lords,
                                               For they'll see them no mair.

That event was, I think, in the fifteenth century, but more recently the sinking of the Titanic inspired a number of folk ballads, and even in my own neighborhood, more recently yet, the Silver Bridge disaster inspired several ballads, unfortunately none of much quality.  Train wrecks in America have been a favorite of ballad tradition.  It is often difficult to distinguish between actual folk ballads and commercial country music, and in fact there often is not a clear line.
    At the end of the 1700's attitudes toward the ballad form began to change.  For one thing, Bishop Percy found a very large manuscript of folk ballads and published them, showing that ballads should not be judged by the cheesy melodrama of the street ballads.  For another thing, the rise of the nation state and the romantic movement turned people's attention toward the "folk,"  that fictional group who are the "real" people of a society as opposed to the international culture of the elite.  Ultimately such ideas helped create the ideology of Naziism, but in the short run the inspired Grimm's fairy tales, and an interest in folk literature, which led to the creation of a new literary form, the literary ballad, which takes the form and manner of the folk ballad but is handled with the intellectual sophistication of a professional poet.
    Robert Southey pioneered the form along with the other first generation romantics.  The most noteworthy example from this group is Coleridge's "Rime of the Ancient Mariner," longer and more sophisticated than any true folk ballad, but with much of the feel.  A little later is Keats' "La Belle Dame sans Merci."  In the twentieth century A.E. Houseman is probably the best representative of the tradition.
    I know of no significant five line stanza except the limerick, which is discussed at as much length as I care to give it in the essay on meter.  I also cannot think of any real six line form, though a possible and obvious pattern comes to mind--ABABCC.  Actually, Shakespeare does use this pattern for his long poem, Venus and Adonis.
    There is one significant seven line form, rime royal.  The rhyme pattern for this form is ABABBCC.  Thus the stanza begins with a quatrain and ends with a couplet.  The fifth line, rhyming with the last line of the quatrian makes a transition from the longer to the shorter pattern, and the last line of the quatrain becomes the first of a couplet.  I am not sure why, but this form comes off much lighter and livelier than one would expect from seven lines of iambic pentameter.  It is well suited to Chaucer's easy and seemingly spontaneous manner, and Chaucer does use it for his book-
length poem, Troilus and Cressida.  Shakespeare uses it for The Rape of Lucrece.  In the twentieth century W.H. Auden uses it for his long Icelandic journal.  His quick and ironic manner, like Chaucer's, is well suited to the form.  Here is an example from Auden:

                                     Professor Houseman was I think the first
                                     To say in print how very stimulating
                                     The little ills by which mankind is cursed
                                     The colds, the aches, the pains are to creating;
                                     Indeed one hardly goes too far in stating
                                     That many a flawless lyric may be due
                                     Not to a lover's broken heart, but flu. 
    For no very apparent reason William Wordsworth chose this form for his almost lugubriously ponderous poem, "Resolution and Independence."  Since the form is not heavy enough for the tone, he then weights the stanza down by making the last line an alexandrine, a line with two extra syllables.  That weights down the last line, and eliminates the usual epigrammatic snap a the end.  The choice of verse form is not an arbitrary matter or whim; each form has its own range of possibilities and limitations.   A poem in a different verse form is a different poem.  For whatever reason, Rime Royal has never become a great favorite in English in spite of its suitibility for long poems, and its general readability.  Here is a retelling of The Lady of the Fountain, one of the romances in the Mabinogion.  It is a poem of over 1,700 lines, and so should give a quite adequate sense of the form.  
    The standard eight line stanza is called ottave rima  This form has a little more weight and dignity than rime royal, though I'm not sure why, maybe because it is longer, or maybe the fact that it has an even rather than an uneven number of lines makes a difference.  The rhyme pattern is ABABABCC.  The fact that it requires two triple rhymes does not make it particularly easy in English.  W.B. Yeats, however, uses it to good effect in "Sailing to Byzantium," quoted one the page, "Six Levels of Rhyme," and Byron shows that it can be made breezy, as well as used for a long poem in his The Vision of Judgement, and his book length Don Juan.  Here is an example from the latter poem:

                                       Juan she saw, and, as a pretty child,
                                       Caress'd him often, such a thing might be
                                       Quite innocently done, and harmles styled,
                                       When she had twenty years, and thirteen he;
                                       But I am not so sure I should have smiled
                                       When he was sixteen, Julia twenty-three,
                                       These few short years make wondrous alterations,
                                       Particulary amongst sun-burnt nations.

Byron is being a bit wicked here, but I don't know whether to be charmed by the innocence he shows (or at least expresses) of the extremes of human behavior, or depressed, considering what has been revealed in recent years about the behavior of some adults toward not only thirteen year olds, but children much younger.
    The nine line stanza form is called the "Spenserian Stanza" after its inventor, Edmund Spenser who created it as a vehicle for his massive allegorical romance, The Faerie Queene.  Rime Royal seems to be his starting point for the form, but he has mutated that form into something that sounds quite different from it.  The rhyme scheme is ABABBCBCC.  The first eight lines are iambic pentameter, the last is iambic hexameter, perhaps the inspiration for Wordsworth ending his rime royal stanzas in Resolution and Independence with an alexandrine.  I admit that I do not understand the dynamics of this line, or why anyone would create it.  It seems too elaborate a pattern to hold conveniently in the head while putting a stanza together, though I suppose by the time Spenser had written the first 10,000 lines or so of The Faerie Queene even so long a pattern must have seemed fairly familiar.  There is apparently something to the form, however, since Keats uses it for a couple of his odes, and other poets have used it for both original poems and translations.
    I don't know a ten line form.  The next pattern I know is one for a complete poem, though it may be made part of a sequence--the sonnet.  Like most of the other forms discussed here, this one is of Italian origin.  I don't know that it began with the poet, Petrarch, but its popularity certainly did.  Petrarch wrote a long sequence of sonnets about his love affair with a woman named Laura, a sort of diary of the emotional ups and downs of his relationship, making it both a "romantic" work and a psychological one.  The Elizabethans liked love poetry, and they loved the idea of a cycle of love poems, so the sonnet caught on in a big way. 
    The sonnet is one of several forms brought back from a diplomatic trip to Italy by Sir Thomas Wyatt, Henry Howard, the Early of Surrey.  It was of special interest to Howard, who wrote a number of sonnets.  He was a poet of limited talent, however, and found the Italian form too taxing, and so developed an easier English form.  To a degree, that is a legitimate move, since English is, in fact, much harder to rhyme than Italian, but I have serious doubts about the validity of the result.  The hardest part of the Italian form is the first eight lines, called the octave, which uses only two rhymes in this pattern:  ABBAABBA.  Usually, there is a break at the end of the fourth line, dividing the octave into two half-stanzas.  The sestet is easier, using either two or three rhymes in any pattern with the restrictions that all the lines have to rhyme, they cannot be all couplets, and the rhymes cannot start closer together and end farther apart.  That leaves several possibilities.  Petrarch's favorite was CDECDE.
    There is normally a sharp break between the octave and sestet indicated by a line break.  Thus, the first part of the poem sets up a situation, problem, or question, and the second part provides a response, a solution, and answer, or a counterpoint.  This seems to me a pattern more naturally suited to intellectual discourse than to love poetry, but it is the form that Petrarch chose, and the Elizabethans were happy to follow him.  It was not until Donne's Holy Sonnets that the idea really took hold that sonnets could as well be used for other purposes than love poetry.  Starting with Sir Philip Sidney's Astrophel to Stella, a sequence of sixty sonnets of high literary quality, the sonnet sequence remained a great favorite for over two decades.  Since the early 17th century, there have been few major sequences, the best known probably being the 19th century Sonnets from the Portugese, by E.B. Browning.
    The English sonnet, as redesigned by Surrey consistes of three quatrains and a couplet:  ABABCDCDEFEFGG.  Thus it replaces a 4/3 ratio, one of the most basic ratios in both art and nature with a 6/1 ratio, which has no natural resonance.  As a result the vast majority of English sonnets go against the grain of the rhyme scheme and break after the eighth line anyway.  Donne in his Holy Sonnets returned to the Italian form, and Milton, Frost and most other good poets have abandoned the English form of the sonnet (Mrs. Browning's sequence is an exception).  It is, however, still a staple of poetry contests.  For one of the very rare cases when the English form really does work with a 6/1 ratio, see Shakespeare's "Sonnet 73" below:
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
In me thou see'st the twilight of such a day
As after sunset fadeth in the west:
Which by and by black night doth take away,
Death's second self that seals up all in rest.
In me thou see'st the glowing of such fire,
That on the ashes of his youth doth lie,
As the deathbed whereon it must expire,
Consumed with that which it was nourished by.
This thou perceivs't which makes thy love more strong,
To love that well which thou must leave ere long.
    Each of the three quatrains develops a single image, and the final couplet brings them together in a logical and satisfying statement.  I would say that this is an almost perfect poem except for the fact that only the first image really seems to have four lines worth of material in it.  Both the others seem padded.  That is yet another difficulty of the sonnet--it is going to be fourteen lines long, come hell or high water.  If there is too much material enough thought and work can often achieve the necessary compression without sacrificing coherence.  If there is too little, it is best to extend the material in new directions, adding more depth and complexity rather than merely padding with more words.  --Jack Hart
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