Front row: Snare drum sounder/relay, Mainline sounders.

Rear row: 2 repeating sounders (sounders with relay contacts - sometimes called transmitters), Polar sounder (responds to polarity reversal rather than on/off current), 3 standard local sounders. Mainline sounders could be used directly in the telegraph line, where the less sensitive local sounders had to be keyed by a relay, except on short local lines.