 Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades.The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were introduced in the late 1930s and remained in use to the 1950s.