Musical form
The term musical form refers to the overall structure or plan of a piece of music, and it describes the layout of a composition as divided into sections. In the tenth edition of The Oxford Companion to Music, Percy Scholes
Percy Scholes
Percy Alfred Scholes was an English musician, journalist and prolific writer, whose best-known achievement was his compilation of the first edition of The Oxford Companion to Music...

 defines musical form as "a series of strategies designed to find a successful mean between the opposite extremes of unrelieved repetition and unrelieved alteration."

Musicology is the scholarly study of music. The word is used in narrow, broad and intermediate senses. In the narrow sense, musicology is confined to the music history of Western culture...

 Richard Middleton
Richard Middleton (musicologist)
Richard Middleton FBA is Emeritus Professor of Music at Newcastle University in Newcastle upon Tyne. He is also the founder and co-ordinating editor of the journal Popular Music.-Education:...

 describes form through repetition
Repetition (music)
Repetition is important in music, where sounds or sequences are often repeated. One often stated idea is that repetition should be in balance with the initial statements and variations in a piece. It may be called restatement, such as the restatement of a theme...

 and difference: difference is the distance moved from a repeat; a repeat being the smallest difference.

"Form is a theatrical event of a certain length, and the length itself may be unpredictable."

Christian Wolff, quoted in Aspects of 20th Century Music, ISBN 0130493465

"There is an idea, the basis of an internal structure, expanded and split into different shapes or groups of sound constantly changing in shape, direction, and speed, attracted and repulsed by various forces. The form of the work is a consequence of this interaction. Possible musical forms are as limitless as the exterior forms of crystals."

Edgard Varese, quoted in Aspects of 20th Century Music, ISBN 0130493465

"'Form' has always come into being in a dialogue between particular 'instances' and the larger body of work, or 'tradition.'"

Richard Middleton (1999). Form. Key Terms in Popular Music and Culture, p.141. Malden, Massachusetts. ISBN 0631212639.