Decoder
A decoder is a circuit in the panel switching system that acts as a type of routing table. It accepts input from a sender in the form of a 3-digit office code, and returns a set of instructions that tells the sender how to route the call towards its destination office[1]. Each call must be routed individually so there is one decoder operation per call. An average-sized panel office usually has three decoders. The holding time for a decoder is about 300 milliseconds.
In later crossbar switching systems, the decoder evolved into a marker.