A general guideline or approximate method for doing something, based on practical experience rather than exact measurement or scientific precision.
This phrase dates to the 17th century and likely comes from the practice of using one's thumb as a rough measuring tool - the thumb's width is approximately one inch. Contrary to popular myth, it has no connection to domestic violence laws, but rather to practical craftsmanship and measurement techniques.
The human thumb is remarkably consistent as a measuring tool - for most adults, the thumb width equals about one inch, and the distance from thumb tip to first joint is roughly one inch as well. This made thumbs invaluable for craftsmen before standardized rulers were common, leading to surprisingly accurate measurements in pre-industrial construction and crafts.
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