[*] 608. Rhythm consists of the division of musical sound into equal intervals of time called Measures or Feet. 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 (). [*] b. A long syllable is regularly equal to two moræ, and is represented by the sign ¯, or by the quarter note or crotchet (). [*] c. A long syllable may be protracted, so as to occupy the time of three or four moræ. Such a syllable, if equal to three moræ, is represented by the sign (or dotted quarter ); if equal to four, by (or the half note or minim, ). [*] 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 moræ by the sign [macrcirc]. [*] g. One or more syllables are sometimes placed before the proper beginning of the measure. Such syllables are called an Anacrūsis or prelude. 1 The anacrusis is regularly equal to the unaccented part of the measure.