The A hardwired electromechanical key telephone system (KTS) comprising components physically wired together by hand. Developed by the Bell System and first marketed in 1953, 1A1 systems superseded the 1A systems, adding a few features, including line status lamps that lit steadily to indicate a line in use and flashed to indicate a line on hold. 1A1 systems were superseded, in turn, by 1A2 systems. See also 1A, 1A2, and KTS.