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In Greek and Latin meter, an anceps syllable is a syllable in a metrical line which can be either short or long. An anceps syllable may be called "free" or "irrational" depending on the type of meter being discussed.
Anceps syllables can be found in a variety of metrical forms. Anceps syllables in the middle or beginning of a line are characteristic of Aeolic meters, such as the Sapphic meter. These initial anceps syllables may reflect earlier Indo-European meters where only the last few syllables of a line were regulated. Anceps syllables are also found in the iambic trimeter of Greek drama.
Note that the anceps is distinct from the phenomenon of brevis in longo. The possibility of brevis in longo is found universally across all meters, while the anceps is found only in particular verse forms. Also, a brevis in longo is always felt to be long, while the anceps may be short or long freely. Monosyllables are always anceps in English — they can take a stress or shrug it off, no matter how "important" they may seem.