Size Of Writing Paper
The sizes of English laid papers were commonly indicated by watermarks, so no matter what size a sheet was cut down to, the original size could still be identified.
Post had a bugle, copy a fleur-de-lis; foolscap a lion rampant or Britannia, and pott the English arms.
There’s only one type of rectangle that has this ability.
Because these half sheets have the same proportions as A4, they also have a name – A5.
However, A4 side lengths aren’t simple numbers like 200 or 300 millimetres. If you take a sheet of paper and cut it halfway down the longer side, you end up with two new pieces of paper.
These pieces of paper each have half the area of the original sheet, but they are the same proportions as the original sheet!
It’s probably the most common size of paper and it’s used in most countries.In the ISO paper size system, the height-to-width ratio of all pages is the square root of two (1.4142 : 1).In other words, the width and the height of a page relate to each other like the side and the diagonal of a square.By the way: The Japanese JIS P 0138-61 standard defines the same A series as ISO 216, but a slightly different B series of paper sizes, sometimes called the JIS B or JB series. For example, JB3 is 364 x 515, JB4 is 257 x 364, and JB5 is 182 x 257 mm. It introduces additional magnification factors and is not an international standard.The following table shows the width and height of all ISO A and B paper formats, as well as the ISO C envelope formats.The C series of formats has been defined for envelopes. For example, the geometric mean of the numbers 2 and 8 is 4 (because 4/2 = 8/4), whereas their arithmetic mean is 5 (because 5-2 = 8-5).The arithmetic mean is half-way between two numbers by addition, whereas the geometric mean is half-way between two numbers by multiplication."Cartridge paper" was sold in 53-inch wide rolls rather than sheets.Some of these named sizes continue in use, although the present-day dimensions are slightly different, perhaps reflecting trimming: double elephant, 40 inches × 27 inches, 1020 mm × 690 mm; royal, 20 inches × 25 inches, 510 mm × 675 mm; imperial, 22 inches × 30 inches, 560 × 760 mm.This aspect ratio is especially convenient for a paper size.If you put two such pages next to each other, or equivalently cut one parallel to its shorter side into two equal pieces, then the resulting page will have again the same width/height ratio.