The workings of the principle of Unity (to which attention must first be given, because it appears to come first in the order of creation) are shown in the following elementary details of composition:—
(1) Music is not an art that deals with space, but with Time; therefore the units of its metrical structure are not inches and the like, but divisions of time, the basis of which is the beat. The principle of Unity dictates that the beats which are associated in one and the same musical sentence shall be of equal duration. Every musician admits the necessity of keeping "strict time"—that is, marking the beats in regular, equal pulses. The sub-divisions of the beats (for example, the eighth or sixteenth notes within a beat) must also be symmetric. So imperative is this law that it generally prevails through the entire piece, with only such temporary elongations or contractions (marked ritardando or accelerando) as may be introduced for oratorical effects.
(2) The beats are grouped in measures of uniform duration; that is, containing equal numbers of beats.
(3) The natural accent falls upon the corresponding beat, namely, the first, of each measure; therefore it recurs regularly, at uniform intervals of time.
(4) The melodic contents of the first measure, or measures, are copied (more or less literally) in the next measure, or measures; and are encountered again and again in the later course of the piece, thus insuring a fairly uniform melodic impression from which the character and identity of the composition are derived. Turn to the 8th Song Without Words of Mendelssohn, and observe how insistently the figure
[Illustration: first fragment of 8th Song]
and its inversion