Small, curved tube that used to be added to brass instruments to change the length of the tubing before the invention of valves. There were often a set of crooks so the instrument could play in several keys.See bocalthe detachable part of the tube on wind instruments such as horns and trumpets. it is made in different sizes to give a different basic key to the instrument. today valves are generally used instead<br><br>Loop of tubing for an instrument such as a french horn. Terminal crooks fit into the instrument (or a coupler) at one end and receive the mouthpiece (or a tuning bit) at the other. The choice of crook or shank determines the key in which the instrument will play