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In railway signalling, an Annett’s key or Annett key is a large key that locks levers or other items of signalling apparatus, thereby serving as a portable form of interlocking. When not in use, the key is normally held in an Annett's lock (or Annett lock) that is fixed to the lever or concerned apparatus.
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, 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. Once the apparatus was unlocked, the key could be taken to the controlling signal box, where it would be used to allow setting another apparatus. Once the block was clear the key would enforce the reverse sequence of operations.
In the Scottish Region Tokenless Block system of signalling, Annett's keys are used as "shunting keys".
- Foster, Richard D.: "A Pictorial Record of L.N.W.R. Signalling", page 166. Oxford Publishing Co., 1982 (SBN 86093 147 1).