There is a fairly widespread belief that the term "Rule of Thumb" originally applied to an old English common law that stated a man could beat his wife, provided that any rod used was no thicker than his thumb. Over time "Rule of Thumb" then came to apply to any simple method of approximation.

A fair deal of investigation has gone into the veracity of this origin, and it appears that what we have here is simply another urban legend. The truth of the matter seems to involve the creative interpretation of some comments made by judges in obscure domestic abuse cases, and no codified law has been found describing what a husband should beat his sworn partner in life with. On the other hand, there IS a fair amount of information describing what the brothers and fathers of battered wives have done to abusive husbands throughout the years, and the bludgeons used in such cases were not restricted to any specific size.

The fact of the matter is there isn't any definitive etymology for the "Rule of Thumb" phrase. There are a number of references to carpentry, sewing, and bludgeoning implements, but no certain origin has actually been pinned down.

There is a fairly clear and in depth description of how the "Old English Law" myth arose at http://www.debunker.com/texts/ruleofthumb.html

I have also seen a number of references to an article titled "Rule of Thumb and the Folklaw of the Husbands Stick" by Henry Ansgar Kelly, published in the September 1994 edition of "The Journal of Legal Education". I was unable to track this article down personally.

Not only is the thumb-width rod a load of grotesque bunkum, it's also pretty obvious bunkum. It is, admittedly, provoked by an accident of the English language; or strictly two. Firstly, English is one of only very few Indo-European languages where the words for »thumb« and »inch« are not either one word, or obviously related; the reason is that the ancient measure of the inch, far predating standardized weights and measures, is derived from the length of the upper joint of the thumb. Of course this varies a bit, but before centralized authority, common things like your own body are all you have to work with; see also foot, ell.

The second reason is that »ruler« has increased in its meanings fairly recently. It used to be that a ruler was only the guy who was the boss of all of you; and the wooden instrument with equidistant markings was a rule — in fact, the other usage of the word rule, as »guideline« or »regulation«, is a figurative one springing from the instrument sense; terms such as »slide rule« still bear witness to this circumstance. (In fact, »regulation« is in itself just a way of saying »measurement with a ruler«, since the word rule is related to the Latin »regula«, meaning, you guessed it, ruler. So a regulation is the use or product of a ruler. The figurativity runs so deep it's impossible to even explain myself without tautology.) Unfortunately, the use of »ruler« has now obliterated that of »rule« to such an extent that people no longer intuitively recognize a »rule« as a flat, straight rod with distance markings; and that completes the confusion.

So, in summary, the »rule of thumb« is the top joint of your thumb, the ruler with which you measure out the inches when there's nothing better available. It's imprecise and it's unscientific, but it often gets the job done.

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