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 [Illustration: second fragment of 8th Song] run through the whole number.
(5) The specific figure of the _accompaniment_ is usually reproduced from measure to measure (or group to group) throughout whole sections of the piece.

Observe, in the 37th Song Without Words, how constantly the ascending figure of six tones recurs in the lower part (left hand).
Glance also at No.

30; No.

1; No.25.