Subjects are one to eight measures in length and are composed of motives. Themes are the longest units, from four to several measures long. The motive and phrase often form the basis of the small scale building blocks, or cells , in a composition. Thus, this is a good place to begin. Although phrases can be of any length, the 2 or 4-bar phrase is a standard length found in most music and is used as a point of departure.
Phrases combine to form sentences, and sentences combine to form periods.
Periods combine to form double-periods. The last may combine to form even larger units. Thus, the large scale structure is built from small cells, and the larger structures then echo the form of the small much in the way that a rectangular building resembles the brick from which it is made. This type of aggregated structure is called architectonic form. It is found abundantly in music around the world and resembles the self replicating structures found in fractals and in nature.