The rule of thumb, wow, where to begin. TMS relates that which I think is the most popular definition. Although no written law can be cited, a judge in England is alleged to have ruled that in common law, it was permissible for a man to beat his wife if the rod was no larger than the diameter of his thumb. A contemporary cartoon depicted a robed, wigged and powdered judge carrying bundles of rods which meet that criteria with a cartoon figure of a man switch raised on high chasing a woman. Times have changed and that could and should get you in a lot of trouble and have very serious consequences.
It appears to me that ED D makes a point with his observation that the width of your thumb equals one. One of what was determined about where the thumb was and what was being measured. The rule of thumb can be referenced in many cultures around the world. Nordic, European, Asian and African sources all have references to a rule of thumb. Many very old school measurements were based on the human anatomy ie the distance from the tip of the thumb to the first knuckle, the length of a forearm, the span of a hand, the ratio of the diameter of the thumb to the wrist to the neck the waist etc.
Typically a rule of thumb is a very loose guideline of approach to almost any situation in which some direction is needed. Today, as a rule of thumb, this forum does not welcome posts on politics, religion and desires to avoid bluntly confrontational posts. As rules of thumb, SWMBO still rules, and, if mama ain't happy, ain't nobody happy are a couple of examples
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If you have any other favorite rules of thumb please post them. Many are wierd many are very specific to a particular set of circumstances some are downright wacky and some show really basic wisdom about life.