Binary principle is embodied at a low structural level in the
two-phrase period: the antecedent/consequent idea.
Pieces of music which are divided into two main parts--the second
of which is perceived as an outgrowth of the first--exemplify the
principle. The two main sections are referred to as Part A
& Part B (duh).
The binary principle is expressed above all else in terms of tonal
Part A: typically involves motion from the tonic to an
authentic cadence in a related key (sometimes to a half cadence in
the original key).
Part B: serves as a return, and re-establishes the tonic
before the final cadence.
The interdependence of the two parts rests primarily on motivic
1. two major sections, which may or may not be subdivided into
smaller ones, and
2. the process of movement away from, and back to, the tonic.