forms

A well-formed composition possesses a good balance between similarity and variety. If there is too much similarity in a composition i.e. if there are too many passages that are similar or equal, the composition appears to listeners as monotonic and uninteresting. If there is too much variety in a composition i.e. if there are too many passages that are completely different, the composition appears to listeners as chaotic and hard to follow and understand.

Similarity makes a listener feel unity in a composition. That is, when there are similarities between passages, a listener feels that passages are somehow related and constituents of the bigger whole.

The theory of forms examines certain forms that proved successful through music history because these forms enabled composers to write well-formed compositions. We shall call these forms common musial forms. 

Separate books have been written on the subject of forms.