A poem's meter defines the number of feet in a line as well as its rhythmic pattern. The foot is a single set of syllables in a poem. To determine the meter of a poem, count the number and kind of syllables in a line, as well as their stresses. Then compare these details with examples of different meters given in books on poetry. Meters are classified according to how many beats per line they have: duple, quadruple, sextuple, and so on. A meter is called an "anapest" if it has one stressed and one unstressed beat in every metrical unit (line or stanza). Most poems are based on anapests, but some other meters are used too. For example, a tetrameter is four-beat verse composed of two pairs of anapaests. There are many variations on the basic anapest and tetrameter schemes; for example, an octosyllabic line using only anapaests would be extremely difficult to sustain over more than a few lines. The meter of a poem can also be determined by counting the number of syllables and then dividing this figure by the number of words to obtain the syllable rate. For example, a line of eight syllables written in four-syllable verses would be counted as 32/4=8.25 syllables per line.