ronna-

Blend of r (an arbitrarily chosen initial letter) + Ancient Greek ἐννέα (ennéa, “nine”) + -a (to match the final letter of the SI prefixes from mega- upwards). Coined by Richard J. C. Brown and adopted by the General Conference on Weights and Measures in 2022 as an expansion to the metric prefixes beyond 10^(±24). Some popular unofficial terms already in use were hella-, bronto- and xenna-, but terms beginning with the same letters as existing prefixes were considered undesirable, as were those beginning with common scientific letters such as b or x. Richard J. C. Brown suggested that the new terms begin with r and q, due to their rarity as unit symbols, and that the trends followed by the other prefixes be continued: that they be based on Latin or Greek; that large prefixes end with -a and small prefixes end with -o; that they should be in corresponding large and small pairs; and that the first letters of each prefix should be in reverse alphabetical order (as has been the case for the newer prefixes). He therefore suggested ronna- and ronto- (evoking Ancient Greek ἐννέα (ennéa) and Latin novem (“nine”)), and quecca- and quecto- (evoking Ancient Greek δέκα (déka) and Latin decem (“ten”)), because as 10²⁷ and 10³⁰ when written have nine and ten groups of zeroes, respectively. These were adopted, with quecca- changed to quetta-.