What is the smallest unit of rhythm in a poem?
First, it's important to distinguish between meter and rhythm. A poem's meter is an ideal abstract pattern, roughly like the sound of a metronome in music. Just as the unvaried sound of a metronome is boring, so too poets rarely strive for absolute regularity. Instead, the actual rhythm of a poem diverges from its meter at points. This divergence is called "variation" (or sometimes "syncopation" or "substitution").
The basic unit of poetic meter is the syllable. In English poetics, syllables are divided into two types, stressed and unstressed. The shortest metrical pattern is called a "foot" and consists of two to four syllables, forming a pattern of stressed and unstressed syllables. All of these patterns have names. For example, the two syllable feet are as follows (note that "/" indicates a stressed and "_" and unstressed syllable):
_ / iamb
/ _ trochee
/ / spondee
_ _ pyrrhic
Poetic lines consist of a fixed number of a fixed type of foot. The lines in Frost's "The Road Not Taken" consist of four feet. Although many of the feet are iambs, and thus the meter is called iambic tetrameter, Frost uses many metrical variations. One of the few regular lines of iambic tetrameter is found near the end of the poem:
_ / | _ / | _ / | _ / |
I took | the one | less tra | veled by|