Each line switch in this first, mixed bay has nine lines on nine of its verticals, the tenth vertical being reserved for test purposes. In addition to the 90 lines on these switches, each LLF has at least one simple line switch bay, with ten more line switches carrying 200 lines. Thus the minimum size of a LLF is 290 lines for a line concentration ratio of 2.9:1. Optionally it has still another frame, with ten more switches and another 200 lines, and so forth, up to a maximum line concentration ratio of 5.9:1 since they all shared the same hundred line links. The line circuit is much like that in 1XB with a line relay for alerting the exchange to a trip condition, and the vertical off-normal contacts of the switch vertical serving as cutoff relay.