Crook
An interchangeable curved section of tubing used to retune a natural horn or natural trumpet to a different key.
What it means
A crook is an interchangeable curved section of tubing inserted between the mouthpiece and the instrument to change the overall tube length of a natural horn or natural trumpet, retuning the instrument to a different key.
Musical use
Before the invention of valves in the early 19th century, natural horns and trumpets could only produce the harmonic series of a fixed tube length; to play in different keys, brass players carried a set of crooks of varying lengths and swapped them according to the key of the piece. This technique, sometimes called the invention horn, allowed composers such as Haydn and Mozart to write horn parts in changing keys without a chromatic instrument. The invention of valve technology made crooks largely unnecessary, though they remain in use today in historically informed performance on natural instruments.
Performance practice
When playing a natural horn with crooks, build the crook change into your rehearsal planning — it takes time and also affects the hand position in the bell, which is used to correct many of the natural tones.