What is a musical stop?

A musical stop is a device used in pipe organs that controls the flow of air to specific ranks of pipes, allowing the organist to select different timbres and sounds. Each stop corresponds to a particular set of pipes, and when engaged, it allows those pipes to sound when a key is played. This enables a wide variety of tonal combinations and is essential for creating the rich and diverse sound characteristic of organ music. The term can also refer to similar mechanisms in other keyboard instruments, like the harpsichord.