Poets combine imagery-rich words in patterns of syllables, lines and stanzas to create poems. Sometimes a poem seems to ramble in a free-flowing blank verse that does not include rhymes. When the poet does incorporate a pattern of rhyming words within the structure of the poem, students are assigned to discover it. A poem's pattern of rhyming words is called its rhyme scheme.
The rhyme scheme of any poem can be expressed like a code by a series of letters. When a poem contains an external rhyme scheme, the rhyming words will all occur at the end of the lines. The last word of the first line is assigned the letter "a". Every time the last word of another line rhymes with that word, it will be assigned the same letter, "a." Each new rhyme will receive the next letter in the alphabet. So, a four line poem, in which the alternate lines rhyme, will have the following rhyme scheme: abab.
When every other line in a four line verse rhymes, the rhyme scheme is a cross rhyme. This is written: abab. However, when the first and fourth lines rhyme and the two middle lines also rhyme, the rhyme scheme is an envelope rhyme which is written: abba. When nothing rhymes, the poem is written in blank verse. Blank verse and free verse are the same thing.
Certain classic poetic forms always follow particular rhyme schemes. Each verse in a ballad will have a rhyme scheme in which the even numbered lines rhyme. This can be expressed as "xbyb". Limericks have five lines in which the first, second and fifth lines rhyme, so their rhyme scheme is "aabca." While all sonnets contain 14 lines, the distinctions between Shakesperean, Spenserian and other sonnets are discerned by their rhyme schemes.
Most often, the rhyme scheme of a poem refers to its external rhyme, the rhymes made by the last words of each line. Occasionally, poems also employ an internal rhyme scheme so that words from within the lines rhyme with each other. The best known example of a poem that has an internal rhyme scheme is Edgar Allan Poe's "The Raven." This poem has both an internal rhyme scheme and a more recognizable external rhyme scheme.