- " (a double prime)
- (For example, six inches can be symbolised as either 6in or 6").
- Length / distance
Primarily used in the United States, Canada and the United Kingdom.
Since 1959, the inch has been defined and internationally accepted as being equivalent to 25.4mm (millimetres).
The inch has been used as a unit of measurement in the United Kingdom since at least the seventh century, and in 1066 was defined as being equal to the length of three dried barleycorns placed end-to-end (a definition which survived for several centuries).
In the 12th Century the Scottish inch was defined as being equivalent to the width of an average man's thumb at the base of the nail. Similar units of measurement existed in many areas of what is now modern Europe, with the word for inch in Portuguese, French, Italian, Spanish and numerous other languages being the same or very similar to the word for thumb.
The English word inch derives from the Latin uncia, meaning one-twelfth part (an inch traditionally being 1/12 of a foot).
Even in the twentieth century various definitions of the inch were still applied around the world, although these differed by less than 0.001%. In 1930 the British Standards Institution adopted an inch of exactly 25.4mm, with the American Standards Association doing likewise in 1933, and the first country to legally adopt this definition was Canada in 1951.
In 1959 the United States and British Commonwealth countries signed a treaty agreeing to the standardised 25.4mm definition.
- A United States quarter (25 cent) coin is just under one inch in diameter.
- A fully-grown human eyeball is roughly one inch in diameter.
In 1995 in the UK the inch (along with the foot, yard and mile) was officially stated as the primary units of measurement for road signs and related measurements of distance and speed. In other contexts metric measurements are now the primary system, although inches are still often used informally, particularly by people who were born and educated in pre-decimal Britain.
In the United States, surveyors use the U.S. Survey inch, defined as 1/39.37 of a metre, derived from the Mendenhall Order of 1893 that equated 1 foot with 1200/3937 meters.
- The inch is traditionally the smallest whole unit of length measurement in the imperial system, with measurements smaller than an inch being stated using the fractions 1/2, 1/4 , 1/8, 1/16, 1/32 and 1/64 of an inch.
- In the UK in the early 19th century precision engineers started to use one thousandths of an inch as greater measuring accuracy became possible, and multiples of this new fraction subsequently became known as a thou.