Why December means ten and a centipede never has a hundred legs
December is the twelfth month, yet its name says tenth. Centipedes rarely have anywhere near a hundred legs. English is packed with number prefixes borrowed from Latin and Greek, and they quietly count the wheels on a tricycle, the sides of a hexagon and the decades in an octogenarian's life.
Most European languages draw on two main systems of numerical prefixes, one Latin and one Greek, each with subsystems, while Sanskrit plays a marginal role. That is why English can say both quadruped and tetrapod, or bicycle and dipod. The shared heritage spread widely: Romance languages inherited the roots, Norman French carried many into English, and nineteenth and twentieth century coinages were borrowed across languages, so the same prefixes pop up almost everywhere.
The prefixes follow different kinds of number. Cardinal forms count items, adverbial forms such as once or twice count occurrences, distributive forms originally meant so many each, and ordinal forms give position. One suffix can serve more than one category, so secondary and tertiary are ordinal while binary and ternary are distributive. For hundreds there are rival versions built on a Latin root in gent and on centi. Many of the longer forms in reference tables are really one-off inventions; scientists prefer scientific notation or SI prefixes for huge and tiny quantities.
Some familiar words mislead. Centipede, millipede and myriapod hint at a hundred, a thousand and ten thousand feet without meaning it literally. Millennium is not built from milli but shares its Latin root, mille. Several metric prefixes are not number words at all: mega has no numeric origin, and peta is a pun on penta. The larger ones, from peta up to quetta, borrow Greek or Latin numbers from five to ten to mark the fifth through tenth powers of a thousand.
Chemistry breaks the pattern too. Systematic names mostly use Greek prefixes, but nona for nine is an exception, and organic chemistry's names for one to four carbons, meth, eth, prop and but, come from older compound names rather than numbers. Anniversaries lean on the same toolkit, from a centennial at 100 years to a sesquicentennial at 150 and a bicentennial at 200.
Source: Numeral prefix
