When rhythm is regular, it is often called meter. Each verse is made up of a number of metrical feet. Use a pair of terms to describe a line of verse: first, an adjective for the basic kind of foot. Typical feet include iambs (the most common in English poetry), trochees, and spondees. Less common are dactyls, anapests, and amphibrachs.

The second term gives the number of feet in each line. The most common in English are pentameter (five beats per verse) and tetrameter (four beats per verse); other possibilities are monometer, dimeter, trimeter, and hexameter.

No meter is perfectly regular. Apart from the theoretical problem that no two syllables will receive precisely the same stress, most poets (even the most apparently regular) try to vary their verse by introducing occasional metrical substitutions.

From the Guide to Literary Terms by Jack Lynch.
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