Traditionally, folding rules for carpenters were made of boxwood, with
a clear finish. It gives a yellowish appearance. There are less
expensive folding rules produced in white (also made of boxwood) and
there are rules made in white for masons. The mason's rules, quite
naturally, are to be differentiated by their markings, which are
pertinent to masonry layout. In a general sense, yellow and clear
finished rules are for carpenters (with some exceptions) and white
rules are for masons and engineers (also with some exceptions).