1

There are many numerical words, such as:

Primary, Secondary, Tertiary

Pentagon, Hexagon, Heptagon

Billion, Trillion, Quadrillion

Unity, Duality, Trinity

Methane, Ethane, Propane

How are the prefixes chosen? Why are there so many prefixes for each number? For 1, there is:

  1. Prim-

  2. M-

  3. Un-

  4. Meth-

And that's only some of them. When do you use which prefix? This has always confused me.

3

Why are there so many? Because English is partially derived from Romance languages that use both Latin and Greek root words, and Latin and Greek had a lot of numerical prefixes.

Primary, Secondary, Tertiary: These are Latin ordinal prefixes.

Pentagon, Hexagon, Heptagon: These are Greek cardinal prefixes.

Billion, Trillion, Quadrillion: These are Latin cardinal prefixes but use the multiple prefix for 2.

Unity, Duality, Trinity: Latin cardinal prefixes again.

Methane, Ethane, Propane: IUPAC nomenclature. The first four are derived from existing chemical names and they then switch to Greek cardinal prefixes.

As to how to choose...you just have to learn them. You can try to learn the sequences from that wikipedia link, but it's not unusual for a series to switch system partway through, as your examples demonstrate.

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