Discussion of what size external disk drive to buy (I’m thinking of a 500Gb model to put an end to data squeeze for a good long time) lead to a question about the meaning of the SI prefix tera-. Wikipedia’s SI prefix page explains that the prefixes started from Greek roots for large things, then switched over to numbers as sources:

• giga- (G, 10⁹): From the Greek root for giant.
• tera- (T, 10¹²): From the Greek root for monster.
• peta- (P, 10¹⁵): Since tera- looks like tetra- (Greek for four with a letter missing), peta- is from penta-, Greek for five with a letter missing.
• exa- (E, 10¹⁸): From the Greek prefix hexa- (six), with a letter missing.
• zetta- (Z, 10²¹): From the Latin septum (seven), with the p dropped, and the first letter changed from s to avoid confusion with existing SI symbols.
• yotta- (Y, 10²⁴): From the Greek octo- (eight), with the c dropped and a y added to avoid having a symbol of O, indistinguishable from zero.

The official Bureau International de Poids et Mesures page about the prefixes is interesting because of its links to the actual resolutions that created the prefixes. Unlike ISO, BIPM manages to make their official output refreshingly brief: their resolutions are only a few paragraphs at most.

One aspect of this I don’t understand: why the BIPM approves new prefixes in such small increments. Peta- and exa- appeared in 1975, and zetta- and yotta- in 1991. Why dole them out so slowly? Of course, there are proposals, including one that goes all the way to 10⁶³ (luma-).

The scientists take exception to the computer geeks using their decimal prefixes for binary amounts. 2²⁰ is not 10⁶, and the inaccuracy gets quite large by the time you are discussing exabytes of data. There is a system of binary prefixes (gibibytes, anyone?), but they have not caught on yet, and sound silly, so I don’t think they ever will.