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In British railway signalling, an Annett's key or Annett key is a form of trapped key interlocking 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.