In the early days of telephony, through roughly the 1960s, companies used manual telephone switchboards, and switchboard operators connected calls by inserting a pair of phone plugs into the appropriate jacks. Each pair of plugs was part of a cord circuit with a switch associated that let the operator participate in the call. Each jack had a light above it that lit when the telephone receiver was lifted (the earliest systems required a generator on the phone to be cranked by hand). Lines from the central office were usually arranged along the bottom row. Before the advent of operator distance dialing and customer Direct Dial (DDD) calling, switchboard operators would work with their counterparts in the distant central office to complete long distance calls. With the development of computerized telephone dialing systems, many telephone calls which previously required a live operator can be placed automatically by the calling party without additional human intervention. Switchboard operators are typically required to have very strong communication skills.