KOB stands for "key on base". A KOB is a combination unit consisting of a key and a relay or sounder, or both. On the left is a Civil War era unit consisting of a camelback style key and a Barclay box relay. The box amplified the clicking so it also acted as a sounder (similar in operation to the snare drum sounder). The unit on the right is a pretty common KOB with a local sounder and standard Triumph style key - usually used in smaller local branch offices.