Other forms: binary forms
A piece of music in binary form stops after a first section and starts a second section with a different feel. It can switch from fast to slow, from a minor key to major, or from bouncy to smooth and flowing.
Binary form was common in classical music from the 17th to the 19th centuries, and can be found in works by Bach, Scarlatti, and others. Typically, the first section is repeated before starting the second — which is then also repeated, giving a structure of AABB. In some pieces, after starting differently, the second section begins to sound more like the first section again. In the 1800s, that tendency led to ternary form, a three-part composition with structure ABA.