Kai se tuuma on jo jostain 1700-1800-luvulta asti ollut määritelty 2.54 senttimetriksi. STRO-kirjan tuumamitat korreloivat 2.54 cm:n kanssa.
http://www.unc.edu/~rowlett/units/
"Late in the nineteenth century, after both Britain and the U.S. signed the Treaty of the Meter, the foot was officially defined in terms of the new metric standards. In the U.S., the Metric Act of 1866 defined the foot to equal exactly 1200/3937 meter, or about 30.480 060 96 centimeters. This unit, still used for geodetic surveying in the United States, is now called the survey foot. In 1959, the U.S. National Bureau of Standards redefined the foot to equal exactly 30.48 centimeters (about 0.999 998 survey foot). This definition was also adopted in Britain by the Weights and Measures Act of 1963, so the foot of 30.48 centimeters is now called the international foot.
"The inch is a traditional unit of distance equal to 1/12 foot or exactly 2.54 centimeters. The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. The inch was originally defined in England in two ways: as the length of three barleycorns laid end to end, or as the width of a man's thumb at the base of the nail. The barleycorn definition is peculiarly English, but the thumb-width definition is generic. In fact, in many European languages the word for inch also means thumb: examples include the Dutch duim, Swedish tum, French pouce, and Spanish pulgada. In the history of English units the inch seems to come before the foot: after the Norman conquest of 1066 the foot was defined to equal 12 inches, rather than the inch being defined as 1/12 foot."