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The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency.
The standard defines the "A", "B" and "C" series of paper sizes, which includes the A4, the most commonly available paper size worldwide. Two supplementary standards, ISO 217 and ISO 269, define related paper sizes; the ISO 269 "C" series is commonly listed alongside the A and B sizes.
The international paper size standard is ISO 216. It is based on the German DIN 476 standard for paper sizes. Each ISO paper size is one half of the area of the next larger size in the same series. ISO paper sizes are all based on a single aspect ratio of the square root of 2, or approximately 1:1.41421. There are different series, as well as ...
Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 1.7 × 10 5 mm 3) [33] and short-term stability. They are used in many commercial, portable and aerospace applications. Modern rubidium standard tubes last more than ten years, and can cost as little as US$50.
[citation needed] A4 ("metric") paper is easier to obtain in the US than US letter can be had elsewhere. [citation needed]. The ISO 216:2007 is the current international standard for paper sizes, including writing papers and some types of printing papers. This standard describes the paper sizes under what the ISO calls the A, B, and C series ...
Although metric, based on the A4 paper size, and named to suggest that it is part of the official ISO 216 paper sizes, it is only a de facto standard. It is often referred to as (metric) "foolscap" or "folio" because of its similarity to the traditional foolscap folio size of 8 + 1 ⁄ 2 in × 13 + 1 ⁄ 2 in (216 mm × 343 mm).
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A quire of paper is a measure of paper quantity. The usual meaning is 25 sheets of the same size and quality: 1 ⁄ 20 of a ream of 500 sheets. Quires of 25 sheets are often used for machine-made paper, while quires of 24 sheets are often used for handmade or specialised paper of 480-sheet reams.