[99.] Measures are usually classified as simple and compound. A simple measure is one which has but a single accent, i.e., the measure cannot be divided into smaller constituent groups. There are two main classes of simple measures, two-beat measure, and three-beat measure. A compound measure is (as its name implies) one made up by combining two or more simple measures, or by the elaboration of a single measure (in slow tempo) into several constituent groups. The principal compound measures are four-beat and six-beat, both being referred to as compound-duple measures. Five-beat, seven-beat, nine-beat, and twelve-beat measures are also classified as compound measures.
An English writer[23] classifies measures as duple, triple, or quadruple, specifying that a simple measure is one in which each beat is represented by a note whose value can be divided into halves (
etc.) and that a compound measure is one in which each beat is represented by a dotted-note, whose value can be divided into three parts, (
). There is thus seen to be considerable difference of opinion as to the meaning of the words simple and compound when applied in this connection, the principal question at issue being whether four-beat measure is an individual variety, or whether it is a variety compounded out of two-beat measures, either by placing two of these in a group or by the elaboration of a single measure into a larger number of beats, as is often necessary in slow tempi. Perhaps the easiest way out of the difficulty is to admit that both may be true—but in different compositions. That is, it is frequently impossible to tell whether a composition that is being listened to is in two-beat, or in four-beat measure; and yet it is sometimes possible so to discriminate. Since, however, one cannot in the majority of cases distinguish between two-beat and four-beat measures, it will probably be best to leave the original classification intact and regard four-beat measure as a compound variety.
100. The commonest varieties of measure are:
1. Duple (sometimes called even measure, or even time), in which there are two beats, the first one being accented. Examples of duple measure are 2/4, 2/8, 2/2, two-quarter,[24] two-eighth, and two-half measure, respectively.