Dial Tone Marker
A dial tone marker (DTM) is a type of marker in a No. 5 Crossbar telephone switch. Its single job is to connect a calling subscriber to an originating register at the moment they take their phone off hook. Dial tone markers do not supply dial tone themselves. They are named that way because the customer getting a dial tone is the ultimate result of the marker's action.
History
Dial tone markers did not exist in early versions of the 5XB. Their job was carried out by combined markers, itself a retronym. Before dial tone markers were separate circuits, all markers took care of all nine types of jobs: dial tone, intra-office, outgoing, incoming, through tandem, through toll, inter-marker, reverting, and pulse conversion. By 1953, it was found that cost could be reduced substantially if dial tone jobs were given to separate circuits, which freed up the remaining markers for the remaining eight jobs only.
Although the number of total markers was increased by the addition of separate dial tone markers, the complexity of each type of marker was reduced.