Short answers, pulled from the story.
The inch was officially defined as exactly 25.4 millimeters on the 1st of July 1959. This decision followed a recommendation by the Commonwealth Science Congress in 1946 and marked the end of centuries of variation between the United States and the United Kingdom.
Before 1843, the legal definition of the inch relied on three grains of barley placed end to end. A statute from 1324 under King Edward II of England legally defined the inch as three grains of barley, dry and round, laid lengthwise.
Before 1959, the United States and the United Kingdom measured the inch to slightly different lengths, creating a discrepancy of less than one ten-thousandth of an inch. By 1893, the US inch was effectively defined as 25.4000508 millimeters, whereas the UK inch was 25.399977 millimeters.
The US survey inch was phased out on the 1st of January 2023 to eliminate a 2 millionth part difference between the standard and US survey inches. The National Institute of Standards and Technology announced this change to supersede the survey foot with the international foot equal to 0.3048 meters exactly.
The English word inch was an early borrowing from the Latin uncia, meaning one-twelfth or Roman inch. The consonant change from the Latin c to English ch is palatalisation, and the word is cognate with ounce.