The TLF having twice as many links, junctor switches, and junctors as the LLF, there are always twice as many LLFs as TLFs. As first designed, the maximum number was ten TLFs and twenty LLFs, known as 10x20, and at first rarely achieved. In the late 1950s multiple trunk junctor switch bays (ETL and SETL) were added to give each TLF access to more junctors. The first expanded version allowed each office to have 20x40, and in the 1960s the maximum reached 30x60. Development stopped at that point because the four-stage layout was becoming progressively less efficient at greater sizes, and because the 1ESS switch with eight stages was under development.
