# What is the difference between "Hept-" and "Sept-" prefixes?

As I understand it, both the prefixes "Hept-" and "Sept-" are used to indicate seven of something.

We have examples of English words that use both:

e.g.

Heptathalon, Heptagon, Heptane

vs

Septuagenarian, September

I don't believe this is the case for other numbers- "Quad" is four, "Pent" is five, "Hex" is six, etc...

I believe "Sept" comes from the word "septem", which is Latin for "seven". So where has "Hept" come from? And why do no other numbers have an equivalent?

• Heptember sounds too silly. Aug 24, 2015 at 22:48

It's the difference between Latin and Greek. Four, five and six do have their own forms.

Latin: bi-, tri-, quad-, quin-, sex-, sept-...
Greek: di-, tri-, tetra-, pent-, hex-, hept-...

Generally, Latin prefixes are added to Latin roots and Greek prefixes to Greek roots.

Tetrahedron, heptagon etc.

• Ah, I see. But 8 is "Oct" for both, is it? (e.g. Octagenarian, Octagon) Jul 24, 2012 at 11:19
• Yes. There is some evidence that some Latin prefixes are themselves derived from Greek. Jul 24, 2012 at 11:22
• Allegedly Hexadecimal was coined even though decimal is a latin root - because someone in 50s thought sexadecimal would be rude
– mgb
Jul 24, 2012 at 13:31
• Actually, you have those prefixes a bit wrong; you seem to have lost a final character on most of them. For example, there is no Latin ∗quad or ∗quin prefix; it’s actually quadr(i) and quint(i), although the latter really should be quinque- more often than it is. You’re also missing letters on the Greek, which should be mono, di, tri, tetra, penta, hexa, hepta, octa, ennea, deca, hendeca, dodeca, trideca, etc. See here for more. PS: this does have both quin and quint.
– tchrist
Jul 24, 2012 at 15:50
• @tchrist. The Latin prefixes are not derived from Greek, Rather the Latin, Greek, English etc. words for the numbers go back to common Indo-European.
– fdb
Aug 26, 2015 at 11:53

The Greek and Latin numeral words are cognates. That means they come from the same source (Lat co-gnatus 'born together').

In particular, they come from Proto-Indo-European, the parent language (dating from around -3000) of modern European languages (excepting only Basque, Finnish, Estonian, and Hungarian).

See this page for details about the derivations of European numeral words. (You'll probably have to rotate it; it's a pdf scan in landscape mode)

"Hept-" is from the Greek "hepta". (See, for example, the Online Etymology Dictionary entry on "heptagon".)

And we do see this distinction with other numbers (though a number of Greek and Latin number terms are cognate). A tetragon, for example, is a four-sided polygon. We also speak of "pentagons" and "quintuplets".