# Serial number

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A serial number is a unique number assigned for identification which varies from its successor or predecessor by a fixed discrete integer value. Common usage has expanded the term to refer to any unique alphanumeric identifier for one of a large set of objects, however not every numerical identifier is a serial number; identifying numbers which are not serial numbers are sometimes called nominal numbers. In data processing and allied fields in computer science, the distinction between serial and nominal numbers is an important one.

Sequence numbers are almost always non-negative, and typically start at zero or one.

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### Applications of serial numbering

Serial numbers are valuable in quality control, as once a defect is found in the production of a particular batch of product, the serial number will quickly identify which units are affected. Serial numbers are also used as a deterrent against theft and counterfeit products in that serial numbers can be recorded, and stolen or otherwise irregular goods can be identified.

Many computer programs come with serial numbers, often called "Compact Disc keys", and the installers often require the user to enter a valid serial number to continue. These numbers are verified using a certain algorithm to avoid usage of counterfeit keys.

Serial numbers also help track down counterfeit currency, because in some countries, each banknote has a unique serial number.

The International Standard Serial Number or ISSN seen on magazines and other periodicals, an equivalent to the International Standard Book Number (ISBN) applied to books, is serially assigned but takes its name from the library science use of serial to mean a periodical.

Certificates and Certificate Authorities (CA) are necessary for widespread use of cryptography. These depend on applying mathematically rigorous serial numbers and serial number arithmetic

The term "serial number" is also used in military formations as an alternative to the expression "service number"[citation needed]. In air forces the serial number is used to uniquely identify individual aircraft and is usually painted on both sides of the aircraft fuselage, most often in the tail area, although in some cases the serial is painted on the side of the aircraft's fin/rudder(s). Because of this, the serial number is sometimes called a "tail number".

In the case of the UK Royal Air Force (RAF) the "Serial" takes the form of two letters followed by three numbers, e.g., BT308—the prototype Avro Lancaster, or XS903—an English Electric Lightning F.6 at one time based at RAF Binbrook [1]. During the Second World War RAF aircraft carrying secret equipment or that were in themselves secret had "/G" added to the end of the serial, the "G" signifying "Guard", denoting that the aircraft was to have an armed guard at all times while on the ground, e.g., LZ548/G—the prototype de Havilland Vampire jet fighter, or ML926/G—a de Havilland Mosquito XVI experimentally fitted with H2S radar. Prior to this two-letter, three-number scheme, the RAF and preceding Royal Flying Corps (RFC) utilised a serial consisting of a letter followed by four figures, e.g., D8096 - a Bristol F.2 Fighter currently owned by the Shuttleworth Collection, or K5054 - the prototype Supermarine Spitfire. The serial number follows the aircraft throughout its period of service.