A quatrain is a group of four lines of poetry, or a verse of poetry that has four lines. The quatrain is typically centered around a theme within the poem. A quatrain typically has a specific pattern to the end rhyme of each line. If the poem has multiple quatrains, the rhyme scheme of one may repeat in the others.
Elizabethan sonnets are made up of three quatrains and a couplet (2 lines) at the end, as this sonnet from Shakespeare illustrates (spaces added to show the quatrains):
My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red;
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head.
I have seen roses damasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound;
I grant I never saw a goddess go;
My mistress when she walks treads on the ground.
And yet, by heaven, I think my love as rare
As any she belied with false compare.
From Robert Frost's "Stopping by Woods on a Snowy Evening":
He gives his harness bells a shake
To ask if there's some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
From Coleridge's "The Rime of the Ancient Mariner":
It is an ancient Mariner,
And he stoppeth one of three.
By thy long grey beard and glittering eye,
Now wherefore stopp'st thou me?
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