In railway signalling, an Annett’s key or Annett key is a large key that locks levers or other items of signalling apparatus, thereby functioning as a portable form of interlocking. When not in use, the key is normally held in an Annett's lock that is fixed to the lever or apparatus concerned. With the key removed from the lock, the lever or apparatus is locked and cannot be moved.
The Annett's key takes its name from its inventor, J. E. Annett of the London, Brighton and South Coast Railway. Annett patented his design in 1875; in 1881 that patent was bought out by Saxby and Farmer,[1] one of the principal British companies of signalling contractors at the time.
Annett's keys have been made in a variety of forms. The keys and locks are given a matching configuration to prevent keys of a different configuration being inserted into the wrong lock.
An Annett's key is commonly used to unlock an item of signalling apparatus remote from a signal box, e.g. a ground frame, a set of points, a swing bridge or a turntable.
An Annett's key may be used as a single line token.
In the Scottish Region Tokenless Block system of signalling, Annett's keys are used as "shunting keys".