Inches

One old antrophometic measure unit of length that survives nowadays in different contexts of our lives is the inch.

Differents countries and nations during centuries, mainly in Europe, have used the inch, adapting its origin and evolving according its particular situations, from the Roman Empire.

Thereby, although it had different names in different countries, its believed that its origin is the same, the Latin world "uncia", that meant "one twelfth part" of the Roman "Foot" measure unit. In many other countries evolved as the lenght of the first phalange of the thumb.

There are some old texts (century X) that relate the inch with an old Anglo-Saxon measure unit, the "barleycorns": 1 inch was equal to 3 barleycorn (that was used in Wales and England for measure some rods of metal, the size of the shoes, and other things).

In 1324 the king Edward II enacted the definition of an inch as "three grains of barley, dry and round, placed end to end, lengthwise".

Before the adoption of the international inch, the United Kingdom and most countries of the British Commonwealth defined the inch in terms of the Imperial Standard Yard (1 inch = 1/36 of a yard) . But Canada had its own, different, definition of the inch, defined in terms of metric units. The Canadian inch was defined to be equal to 25.4 millimetres, the amount later accepted as the international inch.

Then, nowadays, the official relation with the International System of Units is:

1 inch = 25.4 mm

And the relation with the US customary/Imperial units is:

 1 inch = 1/36 yd = 1/12 ft

Inches are widely used in many different contexts: screen sizes (it's an important catchword in the mobiles, televisions, and tablets advertisements), pipes widths, hardware (like screws, threads, disk sizes, ...), printers, road signs, measurements of distance and speed, and more.

Finally, the following table lists some different values that has had the inch in different countries along the years and centuries:

Country/Territory	Equivalence with the IS	Note	Obsolete
Amsterdam	≈ 23.5 mm	Mid-19th century	Yes
Austria	≈ 25.8 mm	Mid-19th century	Yes
Bremen	≈ 23.7 mm	Mid-19th century	Yes
Canada,Estados Unidos, Panamá, United Kingdom, International	25.4 mm	The international inch.	No
England	≈ 25.3 mm	Mid-19th century	Yes
France	≈ 27.0 mm	Mid-19th century	Yes
Hamburg	≈ 23.2 mm	Mid-19th century	Yes
Italy	≈ 28.3 mm	Mid-19th century	Yes
Moscow	≈ 27.7 mm	Mid-19th century	Yes
Portugal	≈ 27.0 mm	Mid-19th century	Yes
Rhynland	≈ 26.1 mm	Mid-19th century	Yes
Scotland	≈ 2.5441 cm	Scottish inch: 1/12 ft = 1.0016 inches in imperial units = ≈2.5441 cm	Yes
Spanish	23.22 mm	1/12 ft	Yes
Sweden	≈ 24.74 mm	"Working inch", between 1855 to 1863	Yes
Sweden	≈ 29.69 mm	"Decimal inch", one tenth of the Swedish foot	Yes
Turkey	≈ 31.3 mm	Mid-19th century	Yes

There are some cases that are used the square inches or cubic inches when extends the measurement for 2 or 3 dimensions.


References:

 - http://en.wikipedia.org/wiki/Inch
 - La medida de todas las cosas, 2007