• Go to the first post in this series

Back in 2016, I wound up my third post on the topic of SI prefixes with the words “And that’s all we’ve got so far …” followed by a table summarizing the complete set of SI prefixes at that time. These prefixes are a shorthand way of indicating that some base unit of measurement should be multiplied by some power of ten. So a kilometre is a thousand metres, a microgram is a millionth of a gram, and so on. The range of prefixes was last expanded in 1991, with the addition of the zetta-/zepto- and yotta-/yocto- pairs.

But now the table needs another update, which I’ve provided above, because four new prefixes, denoting extremely large and extremely small quantities, have been adopted under Resolution 3 of the 27th Meeting of the General Conference on Weights and Measures, which took place in November 2022.

These prefixes were proposed in 2019 by Richard Brown, of the National Physical Laboratory in the UK. The need for a formal decision on new prefixes was primarily driven by the potential for massive data generation and storage. The largest prefix within the Système International was (until November 2022) yotta-, denoting 10²⁴ of something—for instance, the Earth’s mass is 5972 yottagrams. But computer scientists were starting to talk about storing 10²⁷ or even 10³⁰ bytes, and they were starting to make up informal names for these quantities. The first of these was hella-, proposed by Austin Sendek in 2010, via an on-line petition. This was derived from a slang expression popular in Northern California at the time—hella, meaning “very”, probably a contracted form of “hell of a”. It was sufficiently popular to be adopted by Google, for a while, though I can’t now get Google to offer anything but “1.0 × 10²⁷ bytes” in response to a search on “1000*yottabytes”.

Round about the same time the prefix bronto- turned up, also indicating 10²⁷, as in brontobyte. The derivation is from the (debated) dinosaur genus Brontosaurus—that is, a very big thing. But the genus name derives from Greek bronte, “thunder”, and sauros, “lizard”, so the etymology of brontobyte gets a bit confusing if you think about it for too long.

There have also been efforts to continue the alphabetical trend suggested by the prefixes zetta- and yotta-, with xenna- (which actually obeys the rules for SI prefix formation!) and xano- (which doesn’t).

Beyond 10²⁷, we start to see prefixes that exist only in the more fanciful corners of the internet, and I won’t get into those.

What Richard Brown noted was that hella- and bronto- get into difficulties when it came to finding a single-letter abbreviation, which needs to avoid duplicating any existing prefix letter, or any of the single letters used for SI units or the additional units accepted by the SI. As large multiples either prefix would be symbolized by an upper case letter, but “H” is taken by the unit of inductance, the henry, and “B” by both the bel and the byte.

A search through the alphabet convinced Brown that the only remaining useful letters, available in both upper and lower case (for multiples and divisors, respectively), were “Q” and “R”. So those were going to be the initial letters of his new units. He also proposed to continue the trend established by peta-, exa-, zetta- and yotta-, which is to reference the power of a thousand that the prefix represents. Peta- is from Greek pente, “five”, (1000⁵= 10¹⁵); exa- from hex, “six”, (1000⁶); zetta-/zepto- from Latin septem, “seven”; and yotta-/yocto- from Latin octo, “eight”. So to form his new prefixes, Brown chose to reference Greek ennea, “nine”, and deka, “ten”. Other factors to consider were the well-established principle that prefixes which reduce the size of the base unit end in “o”, while those that increase it end in “a”; and the emerging trend that multipliers contain a doubled consonant, while diminishers contain a consonant pair (zetta-/zepto-, yotta-/yocto-).

Having put all those requirements together, Brown came up with ronna- for 10²⁷, ronto- for 10^-27, quecca- for 10³⁰ and quecto- for 10^-30. Simple!

But wait a minute. My table at the head of this post shows that the adopted prefix symbolizing 10³⁰ is quetta-. What happened to quecca-? Just one of the things that international organizations need to be careful about, when making up new words. It turns out that queca is a taboo slang word in Portuguese, which would create an amusing effect for Portuguese speakers using Brown’s new prefix. I think I’ll leave you to look the word up for yourself, if you’re so inclined.