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Deca- (International spelling as used by the International Bureau of Weights and Measures; symbol: da) or deka- (American spelling) is a decimal unit prefix in the metric system denoting a factor of ten. The term is derived from the Greek déka (δέκα) meaning "ten".
The prefix was a part of the original metric system in 1795. It is not in very common usage, although the decapascal is occasionally used by audiologists. The decanewton is also encountered occasionally, probably because it is an SI approximation of the kilogram-force. Its use is more common in Central Europe. In German, Polish, Czech, Slovak and Hungarian, deka (or deko) is common and used as a word on its own, always meaning decagram. A runway number typically indicates its magnetic azimuth in decadegrees.
Before the symbol as an SI prefix was standardized as "da" with the introduction of the International System of Units in 1960, various other symbols were more common, such as "dk" (e.g. in the UK and Austria), "D" (e.g. in Germany), and "Da". For syntactical reasons, the HP 48, 49, 50 series as well as the HP 39gII and Prime calculators use the unit prefix "D".
- Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.
- Decimal multiples and submultiples of SI units, 2006, SI Brochure: The International System of Units (SI), 8th edition
- Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811
- HP 48G Series – User's Guide (UG) (8 ed.). Hewlett-Packard. December 1994 . HP 00048-90126, (00048-90104). Retrieved 2015-09-06.
- HP 50g graphing calculator user's guide (UG) (1 ed.). Hewlett-Packard. 2006-04-01. HP F2229AA-90006. Retrieved 2015-10-10.
- HP Prime Graphing Calculator User Guide (UG) (PDF) (1 ed.). Hewlett-Packard Development Company, L.P. October 2014. HP 788996-001. Archived from the original (PDF) on 2014-09-03. Retrieved 2015-10-13.