rule of thumb A rule, or guideline, based on experience and sound judgement rather than scientific knowledge.The origin of the term is vague, but some suggest that it originated in the practice of carpen-ters, seamstresses, artists, and other craftsmen of using their thumb to approximate measurements.The fact that the measurement of an inch is based on the distance between the thumbnail and the first joint certainly lends credence to this theory. 74570c18.qxd 9/12/07 12:39 AM Page 424 |