The phrase, "rule of thumb" refers to a rule or guideline that is usually though not always correct, it lacks strict precision.
Its origin is very simple; it is based on the use of one's thumb as a measuring tool that, of course, can only give approximate measurements. Most Old English measurements originated from the bodily dimensions of the king such as the length of the "foot" for one foot, the inch (thumb tip to first knuckle), cubit (elbow-to-fingertip), and yard (nose-to-fingertip).
There is a second origin given for this phrase. In early English law, and potentially brought over in colonial American, a man was allowed to discipline his wife by beating her provided he used a stick no greater than the diameter of his thumb
Rule of thumb was a term, a miller would use when grinding flour.
He would rub the flour between his thumb and forefinger, if it was too course he would lower the grinding stones to make the flour finer.
heuristics
In the early centuries it was acceptable for a man to hit his wife if she did something wrong. In order to rule the beating as non abusive, the husband had to use a stick that was no bigger than his thumb. That then gave way to the term, "rule of thumb."
There is no left handed rule. The correct term is right handed rule. Using the right handed rule, your thumb is pointing in the direction of current flow, and your fingers are pointing in the direction of magnetic flux flow.
Africa
"Rule of Thumb" means a rule or principle that provides guidance to appropriate behavior.
what does the phrase rule of thumb mean
The rule of thumb is not intended to be accurate or reliable for every given situation. When a person is doing something by the rule of thumb, they are taking an educated guess.
Rules of thumb
A misspelled "Rule of Thumb"
As a 'rule of thumb', 'i' comes before 'e' except after c.
There isn't a rule of thumb in APA format; there are too many rules.
Where did the term derby originate?