A channel from a line to a trunk consisted of three links of switching fabric: line link, junctor, and trunk link. In a 10x20 or larger office, ten channels, numbered 0 to 9, were available from any line to any trunk. The line junctor switch number and the trunk junctor switch number are the same as the channel number. Logic in the marker compares the ten links of each kind to obtain a clear channel. The lack of a channel is called a "mismatch" and resulted in picking another trunk, or another line, or the use of the ALL where that exists, or giving up and letting the caller try again.