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. Master crook and coupler system Early horns had unalterable lengths and permanently attached mouthpieces. This presented problems in concert situations. A different horn was required for different keys, and the instrument could not be tuned. Around 1700 the Leichnamschneider brothers in Vienna developed a horn with a removable mouthpiece that could be connected to a short piece of tubing, called a master crook. Additional pieces, couplers, of different lengths were inserted between the master crook and the body of the horn to change the horn's length, and thus the pitch. Fine-tuning was done with even shorter segments called tuning bits. This simple and relatively inexpensive solution remained in use even into the 19th century. Charles Tully's Tutor for the French Horn, published in London, recommended this system for beginners as late as 1840. The master crook and coupler system presented some problems. In high keys, the instrument was uncomfortably close to the face, while in low keys, so many pieces needed to be added that the instrument could become unstable, adversely affecting the accuracy of the playing. Moreover, the instrument became so long that it was sometimes difficult to reach the bell for hand-stopping, a technique for lowering the pitch of individual notes a semitone or more. This important innovation had been introduced around 1720, and codified by Anton Hampel of Dresden in about 1750, so that the horn could be played chromatically. Inventionshorn To get around these problems Hampel devised a new instrument, the inventionshorn, in which detachable crooks (or inventions) were inserted not in the mouth pipe, but in the middle of the horn. This presented the new problem of fitting the longest and shortest crooks into the same small space. Working with the Dresden instrument maker Johann Werner, Hampel perfected the Inventionshorn sometime between 1750 and 1755. The new horn was capable of the full range of transpositions and quickly became a regular member of the developing symphony orchestra. Fine-tuning of the Inventionshorn remained a problem until J. G. Haltenhof replaced the tenon and socket fittings with slides in 1776. About