sentences:  The switches were designated as A, B, C and D (the paths were A-B-C for outgoing, A-B-C-D-C-B-A for internal and D-C-B-A for incoming).The common control equipment consisted of B- and C-switch selectors, supervisory selectors (a supervisory relay set stayed in-circuit throughout each call), Register Selectors, Registers and Call Control.The most characteristic feature of the exchange's central control unit design was that calls were processed serially.