Why ronna and quetta joined the metric prefixes in 2022
Earth weighs about six ronnagrams and Jupiter about two quettagrams. Those prefixes only became official in 2022, added because data scientists needed bigger words and the alphabet was nearly used up. Of all the Latin letters, only Q and R were still free for new symbols.
A metric prefix sits in front of a unit to scale it by a power of ten. Kilo multiplies a gram by a thousand; milli divides a metre by a thousand. Six such prefixes date from the birth of the metric system in the 1790s, and the full list, now 24 strong, was standardised for the International System of Units through resolutions between 1960 and 2022. Its newest additions, ronna and quetta for the huge and ronto and quecto for the tiny, came from a proposal by British metrologist Richard Brown. Small ones were added mainly to keep the list symmetrical.
Picking symbols was a puzzle. Most letters were already claimed by other prefixes or by units such as N for newton, while capital I, lowercase l and O were ruled out because they look like 1 and 0, and X resembles a multiplication sign. The largest prefixes, from peta upward, loosely echo Greek or Latin number words, with their opening letters adjusted to avoid clashes and the last four marching backward through the alphabet: z, y, r, q.
A few conventions keep things tidy. Multiples use capital letters and fractions use lowercase, except for the early kilo, hecto and deca. Micro alone borrows a Greek letter, mu. Prefixes attach with no space, so ms means millisecond while m s means metre-second. They can never be stacked, which is why the kilogram, the only base SI unit carrying a prefix, yields milligram rather than microkilogram.
Powers of a thousand are preferred, so builders may insist on millimetres over centimetres to avoid a clutter of decimal points. Squaring a prefixed unit squares the prefix too: one square kilometre is a million square metres, not a thousand. Everyday speech bends the rules freely, from megabytes and decibels to wine served in centilitres.
Source: Metric prefix
