[10] For the United Kingdom, guidance on public sector use states that, since 1 October 1995, without time limit, the inch (along with the foot) is to be used as a primary unit for road signs and related measurements of distance (with the possible exception of clearance heights and widths)[11] and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.

[23] One, dating from the first half of the 10th century, is contained in the Laws of Hywel Dda which superseded those of Dyfnwal, an even earlier definition of the inch in Wales.

[24] King David I of Scotland in his Assize of Weights and Measures (c. 1150) is said to have defined the Scottish inch as the width of an average man's thumb at the base of the nail, even including the requirement to calculate the average of a small, a medium, and a large man's measures.

[25] However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material.

[26] In 1814, Charles Butler, a mathematics teacher at Cheam School, recorded the old legal definition of the inch to be "three grains of sound ripe barley being taken out the middle of the ear, well dried, and laid end to end in a row", and placed the barleycorn, not the inch, as the base unit of the English Long Measure system, from which all other units were derived.

[27] John Bouvier similarly recorded in his 1843 law dictionary that the barleycorn was the fundamental measure.

He noted that this process would not perfectly recover the standard, since it might introduce errors of anywhere between one hundredth and one tenth of an inch in the definition of a yard.

In the United Kingdom and most countries of the British Commonwealth, the inch was defined in terms of the Imperial Standard Yard.

[30] In 1893, Mendenhall ordered the physical realization of the inch to be based on the international prototype metres numbers 21 and 27, which had been received from the CGPM, together with the previously adopted conversion factor.

When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions.

Before the adoption of the metric system, several European countries had customary units whose name translates into "inch".

The French pouce measured roughly 27.0 mm, at least when applied to describe the calibre of artillery pieces.