The parallel rules used in drafting offices were about 5 feet long and
5 inches wide.They had long cords trailing out of each end that were
strung around pulleys mounted at each corner of the board (maybe
3'x5'). When properly set up the rules would maintain perfectly
parallel lines all over the board. Set squres were used on the top and
bottom edges to draw various lines. If you have a complete parallel
rule, it should have a lot of cord wrapped around it. Although the cord
is great for stringing dials, it would be a shame to wreck a perfectly
good parallel rule, although they are not used much these days. Those
old rules, primitive as they may be, helped design all the things from
bridges to radios from 1945 to 1980. Drafting machines were also used,
but they didn't have the ability to easily draw long parallel lines
such as those needed for structural steel, although they were good for
detailing parts.