While these letters are used to describe contrasting sections, this contrast is sometimes created by a completely different melody or musical texture, and sometimes by changing the [[key (music)|key]] (or modulating), often by both. A binary form, for example, usually contains very similar musical material in the A and B parts, but in different keys. In a sonata form, a succession of contrasting musical themes and a modulatory plan work together to define the structure of the movement. | While these letters are used to describe contrasting sections, this contrast is sometimes created by a completely different melody or musical texture, and sometimes by changing the [[key (music)|key]] (or modulating), often by both. A binary form, for example, usually contains very similar musical material in the A and B parts, but in different keys. In a sonata form, a succession of contrasting musical themes and a modulatory plan work together to define the structure of the movement. |