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 occur in two situations. First, the final syllables of most metrical lines are anceps, with the idea that a short final syllable can be effectively lengthened by the pause that naturally falls at the end of the line. This is sometimes referred to as "brevis in longa" (short in long). Second, anceps syllables in the middle or beginning of a line are characteristic of Aeolic meters, such as the Sapphic meter. These initial anceps syllables reflect earlier Indo-European meters where only the last few syllables of a line were regulated.