I am often asked about prefixes for other multiples, such as 10 4. 10 5. 10 -4. and 10 -5. The prefix myria- (my-) was formerly used for 10 4. but it is now considered obsolete and it is not accepted in the SI. To the best of my knowledge, no prefixes were ever accepted generally for 10 5. 10 -4. or 10 -5 .
There is a widespread misconception that prefixes for positive powers of ten are all capitalized, leading to the use of K- for kilo- and D- for deca-. Although this does seem like a useful idea, it is not correct.
**The SI Brochure spelling of this prefix is deca-, but the U.S. National Institute of Standards and Technology (NIST) recommends deka-. National variations in spelling of the prefixes are allowed by the SI. In Italian, for example, hecto- is spelled etto- and kilo- is spelled chilo-. The symbols, however, are the same in all languages, so dam (not dkm) is the symbol for the dekameter and km is the symbol for the Italian chilometro.
The prefixes hecto-, deka-, deci-, and
centi- are widely used in everyday life but are generally avoided in scientific work. Contrary to the belief of some scientists, however, the SI does allow use of these prefixes.
The last letter of a prefix is often omitted if the first letter of the unit name is a vowel, causing the combination to be hard to pronounce otherwise. Thus 100 ares is a hectare and 1 million ohms is a megohm. However, the last letter of the prefix is not omitted if pronunciation is not a problem, as in the case of the milliampere. The letter "l" is sometimes added to prefixes before the erg, so 1 million ergs is a megalerg (sounds odd, but better than "megerg").
In computing, a custom arose of using the metric prefixes to specify powers of 2. For example, a kilobit is usually 2 10 = 1024 bits instead of 1000 bits. This practice leads to considerable confusion. In an effort to eliminate this confusion, in 1998 the International Electrotechnical Commission approved new prefixes for the powers of 2. These prefixes are as follows: