Crook (music)
A crook, also sometimes called a shank, is an exchangeable segment of tubing in a natural horn (or other brass instrument, such as a natural trumpet) which is used to change the length of the pipe, altering the fundamental pitch and harmonic series which the instrument can sound, and thus the key in which it plays.
Read more about Crook (music): Master Crook and Coupler System, Inventionshorn, Terminal Crooks, Characteristics of Manual Crooks, The Invention of Valves, Hybrid Instruments