Inches (") are the most commonly used specification units in the American system, such as steel pipes, valves, flanges, elbows, pumps, tees.
In Dutch, the inch (abbreviated as in.) originally meant a thumb, and one inch is the length of a thumb. Of course, the length of a human thumb also varies. In the 14th century, Emperor Edward II issued the "Standard Legal Inches". It is regulated that the three largest grains selected from the middle of a barley ear and arranged in a row are one inch long.
Usually 1" = 2.54 cm = 25.4 mm