A Dictionary of Music and Musicians/Crook
CROOK (Fr. Corps de rechange; Germ. Ton; Bogen). A name given to certain accessory pieces of tubing applied to the mouthpiece of brass instruments for the purpose of altering the length of the tube, and thus raising or lowering their pitch. Since these instruments can only play one scale, the sole method of enabling them to play another is to transpose the fundamental note, and this is done by the crooks. The largest number of crooks is required by the French horn, which is occasionally written for in every key, from the treble B♮ down to A♭ in the bass octave.
The term is also applied to the S-shaped metal tube connecting the body of the bassoon with the reed (Fr. bocale).[ W. H. S. ]