An example of a rule of thumb is the general guideline that you don't wear white after Labor Day.
Origin of rule-of-thumb
- From the use of the thumb as a makeshift ruler or measuring device, as in carpentry
From American Heritage Dictionary of the English Language, 5th Edition
- Michael Quinion lists the first documented use as 1692 while The Oxford English Dictionary puts the first documented use at 1685. Some suggested origins include the fact that the inch originated as the distance between the base of the thumbnail and the first joint, the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air, and the rule of English Royal banquet plate setters using the distance of the thumb to equally space each plate from the table edge. Apocryphally, it has been claimed the term originally referred to the maximum thickness of a stick with which it was permissible for a man to beat his wife, but the earliest use in direct reference to domestic violence may be Del Martin's 1976 book Battered Wives.