The Fountain of Vaucluse--a painting of Petrarch's villa in the south of France--by Thomas Cole (Dallas Museum of Art)
One of the most famous forms of poetry is the sonnet. While most English speakers are most familiar with the Shakespearean sonnet--a form not invented by Shakespeare, though certainly mastered by him--the sonnet was originally created in Italy, where another famous form, the Petrarchan sonnet (similarly not invented by Petrarch, but mastered by him) emerged.
The form is a fourteen line poem in iambic pentameter, with various rhyme schemes. Sonnets are divided into parts of different sizes. If you divide it into two parts of seven, you don't have a sonnet.
The Italian/Sicilian forms are divided into two parts, first part of which is made of eight lines, with the second line having six, with a turn (or volta) that occurs in the second part. A turn is a reversal or shift in the theme, thought, or direction of the poem. As a consequence, the word "however" or "but" or some similar word is not uncommon at the turn.
The Shakespearean sonnet is a little more complex. It is divided into four parts: an introduction of four lines, a thesis of four lines, an antithesis of four lines, and a synthesis in the final two lines. It can also simply have the turn with the final two lines.
The Petrarchan sonnet is made up of two Italian quatrains, abbaabba, which is known as an Italian octave. This is followed by an Italian sestet, cdecde, or a Sicilian sestet, cdcdcd.
The Sicilian sonnet is made up of a Sicilian octave, abababab, and a Sicilian or an Italian sestet.
The envelope sonnet rhymes abbacddc, followed by a Sicilian or an Italian sestet (rhymed, in this case, efgefg or efefef, respectively).
The sonnetto rispetto has a ottava rima, abababcc, or a risotto, ababccdd, followed by either an Italian or a Sicilian sestet.
The Spencerian sonnet has three interlocking Sicilian quatrains, abab bcbc cdcd, followed by a volta and heroic couplet, ee.
The terza rima sonnet contains a terza rima, aka bib cdc did, followed by a turn and a heroic couplet. A terza rima is, as you can see, made up of interlocking Sicilian triplet stanzas.
The English or Shakespearean sonnet is the one most English readers are familiar with. It's made up of three Sicilian quatrains, abab cdcd efef, and a heroic couplet.
The blues sonnet is made up of four blues stanzas and a couplet. The blues stanza is AAa, where AA are identical words, giving us the following form: AAa BBb Ccc Ddd ee.
Naturally, one could come up with a number of other rhyme schemes, and there have been poets who have abandoned rhyme altogether. And while sonnets have traditionally been in iambic pentameter, it's possible to have different line lengths and different rhythms as well. The main restrictions remain the requirement of fourteen lines, an uneven division, and a turn.
Take, for example, the poem The Love Song of J. Alfred Prufrock by T.S. Eliot. It contains two full sonnets, both ending with the same couplet. I'll only discuss the first one, which opens the poem.
First note that the lines are of different lengths: 7, 11, 12, 11, 6, 10, 10, 11, 7, 11, 7, 7, 9, and 8 syllables.
While many of the lines are iambic, including the 10-syllable, making them iambic pentameter, there is some variation in rhythms. And there is a rhyme scheme: aabccddeefgg hh. The scene seems to be a party, and the narrator is proposing to someone that they leave the party; however, in the turn, we have people at the party who are intending to stay.
I'm not going to go into a full analysis. I just wanted to point to how one experienced, Modernist poet used the sonnet form within a longer poem, and did variations on the sonnet form itself.
Finally, one can use sonnets in a series.
The sonnet redoubled uses fifteen sonnets, with the first sonnet providing, line by line, the final line of each of the next fourteen sonnets. In other words, line one of the first sonnet will be the final line of the second sonnet, line two of the first sonnet will be the final line of the third sonnet, etc. To say this is difficult is an understatement, and it's very likely you'll have to go back and forth, massaging each sonnet until they all work and make sense.
With my poem Time For Love and Art, I did my own kind of series of sonnets in which I used the couplet of each sonnet as lines 1 and 3 for the following sonnet, with the final sonnet having a couplet made of lines 1 and 3 of the first sonnet, making it the entire series circular and the sonnets themselves interlocking. I'm sure it's something someone else has probably done, but you never know.
In the end, the sonnet remains a generative form. While Modernist and postmodernist poets complain about the restrictions of the form, I would argue that the restrictions make it highly generative. I certainly have used the sonnet--much more than any other form--and I do not feel restricted or that it's been remotely exhausted. I'm particularly attracted to the Shakespearean sonnet's dialectical movement, as I am myself a dialectical thinker. And while, traditionally, sonnets have been love poems, this has hardly been the case since Baudilaire, and you'll find a great many sonnets on a great many topics.
Indeed, there are still many ways to use and challenge the sonnet form. One could take the forms divisible into four parts and write three quatrains and a couplet on a theme, as I do in my poem Four Displays, or you can even write those four sections as four unconnected poem and create a kind of surrealist or postmodernist juxtaposition that potentially pushes the reader into more complex ways of thinking and making connections.
So, I encourage you to write sonnets. I have used the form too in my teaching, as I explain here. Again, the restrictions are precisely what are generative of good writing and interesting thoughts. And isn't that what we expect from poetry?