Rule of Thumb

SEELEY LAKE – John, reading a financial magazine, might say to his wife Mary, "Honey, it says here as a rule of thumb, we should have an emergency fund set aside equal to six months' worth of household expenses. We don't even have an emergency fund, but we probably should be putting some money aside."

If that was perceived by Mary as a prelude to an unwelcome conversation targeting those expensive new shoes she just bought, she might deflect by saying, "Well, a rule of thumb certainly isn't anything to be ruled by."

Setting aside a possible impending argument between husband and wife, where does the expression "rule of thumb" come from?

Though the expression has been around since at least the 1600s, the explanation is incorrect that traces it back to an apocryphal story about a judge who ruled it was acceptable for a husband to beat his wife (and we certainly hope that isn't how John and Mary's argument ends) as long as the circumference of the stick he used was less than a thumb's width.

Though the exact origin of the phrase "rule of thumb" is unknown, it is highly likely the "rule" had more to do with measurement than with authoritative decrees. Body parts were the first measuring standards. A thumb, from base of the nail to the joint, was about an inch; 12 inches measured about the same as a man's foot, which was used for pacing off distances; the hand unit is still used to measure a horse's height. England's King Henry I defined a yard as the distance from his nose to the thumb of his outstretched hand.

Thumbs were used to estimate many things. For instance, those setting out plates for an English royal banquet used the distance of the thumb to equally space each plate from the table edge. Wetting the thumb then raising it provides an estimate of wind direction. Holding a thumb at the eye-line to cover a faraway object provides a rough estimate of distance or vertical alignment.

Tailors have a handy axiom that states twice the circumference of the thumb is equal to the circumference of the wrist; twice the circumference of the wrist is equal to the circumference of the neck, and twice the circumference of the neck is equal to the circumference of the waist.

The Germans have an expression – pi mal daumen – which literally means the irrational mathematical constant "pi [3.14159265359...] times thumb." Obviously, taking a number that has no ending and multiplying it by a body part that changes slightly with everybody who uses it is never going to result in an exact number. That's why the German expression also translates as "a rough approximation."

However, thumbs don't work very well when discussing money matters or trying to approximate how many people are at an event. For rough estimates of those, a sports idiom works better: ballpark figure. Commentators, especially radio commentators, liked to let their audience know approximately how many people were attending the game. They gave rough estimates, which became known as ballpark figures.

"Well," Mary might say if she is in a conciliatory frame of mind, "can you give me a ballpark figure as to how much you think we'd need to put in an emergency fund? Maybe we can work something out."

Ah, a much happier ending than the stick a thumb's width wide or throwing baseballs (or other things) at one another.