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From The Oxford Companion to Music (Latham, 2011):

Form is “the shape or structure of a musical work.”

“Form can be said to be the way in which the various elements in a piece of music—its pitches, rhythms, dynamics, timbres—are organized in order to make it coherent to a listener…

In his Fundamentals of Musical Composition (written between 1937 and 1948), Schoenberg stated that

form means that a piece is organized: i.e. that it consists of elements functioning like those of a living organism … The chief requirements for the creation of a comprehensible form are logic and coherence. The presentation, development, and interconnection of ideas must be based on relationship’…

Since music is essentially a temporal art, in live performance there is no chance for the listener to rehear a detail or part of a work. This is not the case with painting or literature, for example, where the eye can continue to study a whole work or a detail of it for any length of time. In music a substitute for that property is some kind of repetition. It may be a short sequence of sounds that is repeated, or a long and complex section which most people would find impossible to remember exactly. Nevertheless, the effect of such repetition will be registered, at least by the experienced listener, and will give him or her a sense of order. However, repetition alone would be monotonous. Musical form also consists of the relationship between different patterns of sound.

Works may be classified as single or compound forms. Single forms are formally complete and tonally self-contained and are not divisible into smaller units. Compound forms include two or more single forms; they are usually multi-movement works.”