Rule of thumb

In English, the phrase rule of thumb refers to an approximate method for doing something, based on practical experience rather than theory.^[1][2][3] This usage of the phrase can be traced back to the 17th century and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.

The phrase rule of thumb first became associated with domestic abuse in the late 1970s, when an author mentioned the idiom in an article but did not say that there was any such legal rule. After this, an incorrect belief that there was an actual legal rule spread. The error appeared in a number of law journals, and the United States Commission on Civil Rights published a report on domestic abuse titled "Under the Rule of Thumb" in 1982. Some efforts were made to discourage the phrase, which was seen as taboo owing to this false origin. During the 1990s, several authors correctly identified the spurious folk etymology; however, the connection to domestic violence was still being cited in some legal sources into the early 2000s.