Language ArtsAs is the case with many popular phrases and sayings, it can be difficult to determine the exact origin. The definition of -rule of thumb- is one of those popular expressions. In this article we debunk and explore the actual meaning behind the rule of thumb.

Many people believe the meaning of the popular idiom "Rule of Thumb" comes from an old English common law. They believe the English followed the "rule of thumb" to describe the rule that a man could beat his wife with a rod or stick as long as it was no bigger than his thumb. The law allegedly also transferred to American law. The supposed meaning behind this now-popular expression has since outraged many scholars particularly feminists. However, there are no documented sources to prove that this is where the popular saying came from. Such a law never actually existed in England or anywhere else. Research shows it is not to be found in William Blackstone's treatise on English common law. Blackstone was a British jurist and judge. He is most known for writing the Commentaries on the Laws of England. We can debunk this theory behind "the rule of thumb" because British law since the 1700s as well as laws in America pre-dating the American Revolution prohibit wife beating, even though such prohibition might not have always been that thoroughly enforced.  

Many of the scholars and feminists who often reference this meaning behind the rule of thumb phrase site sources of what was said by the people of the times. However, no law actually existed. One of the earliest written references of the rule of thumb dates back to 1692. It was referenced as a popular phrase during that time, which means it probably goes back to times even further back in history. The actual meaning behind rule of thumb is a lot more literal. The definition actual means to measure something in terms of the size of a thumb. The "rule" is literally a reference to a ruler, not regulation as previously thought. Many methods of measurement originated from body parts. For example, a horse is often measured in hands. A foot measures the length of twelve inches. 

The confusion and incorrect attribution of the rule of thumb meaning could be derived from an inappropriate play on words made by British Judge Francis Buller in 1782. A satirical cartoon was published not long after based on what Buller had supposedly said. There is not much evidence to back up that Buller actually said the words to influence the concept of the cartoon however. It wasn't until 1976 that the traditional phrase rule of thumb became a direct reference to this false legal idea. This idea was further perpetuated in various speeches and articles, which only continued the wrong idea.

http://www.canlaw.com/rights/thumbrul.htm, http://www.worldwidewords.org/qa/qa-rul1.htm