Starting before 1974, Bell prewired new construction houses with 5 or 6 twisted pair cable that would likely meet the specs for a Cat2 or a Cat3 cable if tested.
With the breakup of Bell in the early 1980s, responsibility for prewiring of new construction houses fell to the electricians who mostly used the commonly-available 4-wire, non-twisted, "JKT" cable for telephone prewires.
According to information I've read, that 4-wire cable was designed and intended (by Bell) for useage on retro-fit installations where the wire would be exposed along a baseboard (the non-twisted construction and 4 wires resulted in a small diameter, smooth, perfectly round cable which looked acceptable against a baseboard). Anything that was a pre-wire install (where the cable is concealed in the wall) was to use 5 or 6 twisted pair cable.
Since about 2000, the FCC rules specify that Cat 3 cable is the minimum acceptable for telephone wiring (for new installations).
[This message has been edited by brianl703 (edited 10-11-2005).]
