Poems come in countless forms. There are standardized forms like the sonnet, the villanelle, the sestina or the pantoum, among many others, and there are free verse poems, in which poets don't follow a formula, but develop their own rules according to the poems' intent or argument. Throughout history, poets have bent the rules of poetic forms to serve their poetic needs. The content of a poem often dictates the final form it takes.
Form as Poet's Guideline
Poems often aim to put indescribable emotions, events or conflicts into words. Poetic forms can serve as a starting point. For instance, the pantoum thrives on repetition. Each line in the poem will be repeated, so poets who want to write about issues that nag, linger or haunt could begin their writing process following pantoum rules. When the intent of the poem is more clear, a traditional sonnet might serve a poet better. A sonnet is an argumentative form in which the first two quatrains present a problem or question. Then, a turn or "volta" in the poem offers a resolution in the last six lines.
Form as Mnemonic Device
Many poetic forms were developed in oral cultures so poets or singers could pass stories down to audiences without writing them down. Homer's "The Iliad" and "The Odyssey," along with other epic poems, were written in dactylic hexameter, a poetic form that follows strict metrical and rhythmic guidelines so they could be memorized like songs. The ballad, the ghazal and nursery rhymes are also easily memorized.
Free Verse Poems
Since the 20th century, most published poetry has come in free verse form. Free verse poetry does not follow strict rhythmic patterns or rhyme schemes, and it aims to capture natural speech sounds and patterns. However, poets use other strategies -- line breaks, internal rhyme, repetition or white space on the page -- in this form to heighten their language, intensify their musicality or add beauty to their work. Walt Whitman is one of the most famous American free verse poets.
Form as Irony
Poets sometimes adopt paradoxical poetic forms for their work to heighten the emotional impact on readers. A prime example of this is Sylvia Plath's poem "Daddy." This poem, which hints at the traditional nursery rhyme with repetition and musical sounds, addresses Plath's attempted suicides, and compares her strained relationship with her father to that of a Nazi and a Jew. Readers are more affected by the harsh realities of the poem because the dark content is presented in a childlike rhyme and rhythm.