The Foot is lawfully divided into 12 inches; but there is nothing to prevent it being divided decimally, or otherwise, as convenient.
The Inch is divided according to convenience, either
Sexdecimally, into halves, quarters, &c., down to sixty-fourths. This is the usual division.
Duodecimally, into 12 lines.
Decimally, into tenths and hundredths.
Steel foot-rules usually show all three of these scales.
Some trades may have special scales. Thus type-founders divide the Inch into 6 ‘picas’ each = 2 lines, and the ‘pica’ into 12 points each = 1/6 line or 1/72 inch. Nonpareil type is 6 points; Brevier is 8 points.
2. Standards of the Linear Measures
Tables of measures, from the earliest, about 1500, down to quite recent times, usually began by stating that ‘Three barley-corns make an inch’ or that ‘Geographical measures begin at a barley-corn and increase upward to a league,’ &c.
King David I of Scotland (c. 1150) is credited with the pronouncement that the Scots inch was to be the mean measure of ‘the thowmys of iij men, that is to say an mekill man and a man of messurabil statur and of a lytell man. The thoums are to be messurit at the rut of the nayll.’ But no more in Scotland than in England, or elsewhere, has the inch ever been anything but a division of the foot.