So far we've looked at the type of movement usually found in a good melody, the way its ideas are expressed in phrases, its implied harmony, and various ways that a single small idea can be extended and developed. This first part of the task might be compared to the design of parts for a sculpture or a building. Next comes the task of building these parts into a larger structure.
Most musical compositions are made up of two or three parts. We use the terms binary and ternary to describe two-part and three-part forms.
"Greensleeves" is s very simple binary song that consists of two parallel periods. The sections of the song are labeled A and B. (We use capital letters to refer to formal elements and small letters to refer to phrases.) The first two phrases are a matched pair in which the first has an open ending and the second a closed ending. Then follows part B, a second phrase pair which has much in common with the A section but also introduces contrasting melodic material.
Binary form can be either sectional or continuous. If the A section ends on the tonic, as "Greensleeves" does, it is called sectional binary. If the A section ends on something other than the tonic, usually the dominant, it is called continuous binary. Let's look at another two-part composition. This time the form is continuous binary, which is the more common of the two.