The most natural division of musical time is into measures consisting of either two or three equal parts. But the ancients also distinguished measures of five equal parts.
NOTE: The divisions of musical time are marked by a stress of voice on one or the other part of the measure. This stress is called the Ictus (beat), or metrical accent (see § 611. a).
a. The unit of length in Prosody is one short syllable. This is called a Mora. It is represented by the sign , or in musical notation by the eighth note or quaver ([QUERY]).
b. A long syllable is regularly equal to two morae, and is represented by the sign - , or by the quarter note or crotchet ([QUERY]).
c. A long syllable may be protracted, so as to occupy the time of three or four morae. Such a syllable, if equal to three morae, is represented by the sign llcorner (or dotted quarter [QUERY]); if equal to four, by [QUERY] (or the half note or minim, [QUERY]).
d. A long syllable may be contracted, so as to take practically the time of a short one. Such a syllable is sometimes represented by the sign >.
e. A short syllable may be contracted so as to occupy less than one mora.
f. A pause sometimes occurs at the end of a verse or a series of verses, to fill up the time. A pause of one mora in a measure is indicated by the sign one of two moraeby the sign .
g. One or more syllables are sometimes placed before the proper beginning of the measure. Such syllables are called an Anacr=usis or prelude.[The same thing occurs in modern poetry, and in modern music any unaccented syllables at the beginning are treated as an anacrusis, i.e. they make an incomplete measure before the first bar. This was not the case in ancient music. The ancients seem to have treated any unaccented syllable at the beginning as belonging to the following accented ones, so as to make with them a foot or measure. Thus it would seem that there was an original form of Indo-European poetry which was iambic in its structure, or which, at least, accented the second syllable rather than the first.]
The anacrusis is regularly equal to the unaccented part of the measure.