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 4 years later the Parisian instrument makers Joseph and Lucien-Joseph Raoux, in collaboration with Carl Türrschmidt, came out with the cor solo, a refined version for soloists with crooks in G, F, E, E♭, and D. (Orchestra players needed more crooks, since they needed to play in more keys).
The Inventionshorn design was also applied to other brass instruments. Some examples are a pair of invention trumpets by Michael Saurle (1805) at the National Music Museum. For photographs and a detailed description of the Saurle trumpets (NMM 7131 and 7132), see here (Accessed 21 May 2009).
Read more about this topic: Crook (music)