For postage stamps, separation is the means by which individual stamps are made easily detachable from each other.
Methods of separation include:
In the early years, from 1840 to the 1850s, all stamps were imperforate, and had to be cut from the sheet with scissors or knife. This was time-consuming and error-prone (as mangled stamps of the era attest). Once reliable separation equipment became available, nations switched rapidly. Imperforate stamps have been issued occasionally since then, either because separation equipment was temporarily unavailable (in newborn nations for instance), or to makers of automatic stamp vending equipment (the United States did this in the 1900s and 1910s), as novelties for stamp collectors (particularly when stamps are issued in souvenir sheets), or as errors.
In 1848, Henry Archer patented a "stroke process" for the perforation of stamps, and in 1854 a "rotary process" was patented by William Bemrose and Henry Howe Bemrose. The common aspect of the two processes was the use of rows of small round pins ("combs") to punch out the holes. The processes have been refined since then, but are basically still the ones in use in the 21st century. The key decision for the perforator is the spacing of the holes; if too far apart, the stamps will not separate easily, and the stamps are likely to tear, but if too close, the stamps will tend to come apart in normal handling.
In a few cases the size of the holes has been a factor. In the case of certain stamps produced by Australia for sale in rolls rather than sheets (coil stamps) a pattern can be seen on the stamp's short side of two small, ten large and two small holes.
The standard for describing perforation is the number of holes (or the "teeth" or perfs of an individual stamp) in a 2-centimeter span. The finest gauge ever used is 18 on stamps of the Malay States in the early 1950s, and the coarsest is 2, seen on the 1891 stamps of Bhopal. Modern stamp perforations tend to range from perf 11 to 13 or so.
Stamps that are perforated on one pair of opposite sides and imperforate on the other have most often been produced in coils instead of sheets, but they can sometimes come from booklet panes. Booklet panes can be associated with any combination of one, two or three imperforate sides. Sheet edges can produce any one imperforate side or two adjacent imperforate sides when the stamp comes from the corner of the sheet.
Variations include syncopated perforations which are uneven, either skipping a hole or by making some holes larger. In the 1990s, Great Britain began adding large elliptical holes to the perforations on each side, as an anti-counterfeiting measure.
Rouletting uses small cuts in the paper instead of holes. It was used by a number of countries, but is rarely if ever seen on modern stamps. Varieties, ofen described by philatelists in French terms, include straight cuts (percée en lignes, and percée en lignes colorées with inked cutting bar), arc (percée en arc), sawtooth and the serpentine roulettes (percée en pointe) used by the early stamps of Finland.
A few types of stamps have combined rouletting and perforation, for instance South Africa in 1942.
For the stamp collector, perforations matter, not only as a way to distinguish different stamps (a perf 10 may be rarer and more valuable than a perf 11 of the same design), but also as part of the condition of stamps. Short or "nibbed" perfs are undesirable and reduce value, as are bent or creased perfs. A perforation gauge can be used to measure the perforation that are quoted in perfs per inch.
The self-adhesive stamps in roll or booklet form issued in Australia have "fake" perforations rather than straight-edged cuts as do those in the United Kingdom. This acts as a security measure, being more difficult to fake, and also maintains the 'expected' shape of a stamp to the user.
As is inevitable for a mechanical process like perforation, many things can go wrong. Blind perfs are common, occurring when a hole is not completely punched out, as are offcenter perfs that cut into the design of the stamp, sometimes very badly. Occasionally pairs or larger groups of stamps may be imperforate between meaning that they are not separated on all sides. Although it is very common to have different gauges of perforation horizontally and vertically, in rare circumstances a stamp may have different perforations on opposite sides; in the case of US stamps only a handful of these are known to exist. The various types of perforation errors are collectively known as misperfs.