The Number One Crossbar Switching System, or 1XB switch, was the primary urban local telephone exchange design used by the Bell System in the mid-20th century. Its switch fabric topology was based on the earlier urban panel switch system, which, in turn, was based on the turn of the century Divided Multiple Switchboard. Thus, it had separate incoming and outgoing sections. However, lines appeared only on the Line Link Frame (LLF), rather than requiring a multi-wire connection to two different sections as in a panel switch. The LLF uniting the telephone line circuit at its "column and switch" simplified administration. The first 1XB was the PR7 exchange at Troy Avenue in Brooklyn, New York in 1937.