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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:


Heptathalon, Heptagon, Heptane


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?

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Heptember sounds too silly. – Oldcat Aug 24 '15 at 22:48
up vote 17 down vote accepted

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.

Quadrilateral, quintuplet, September etc.;
Tetrahedron, heptagon etc.

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Ah, I see. But 8 is "Oct" for both, is it? (e.g. Octagenarian, Octagon) – Urbycoz Jul 24 '12 at 11:19
Yes. There is some evidence that some Latin prefixes are themselves derived from Greek. – Andrew Leach Jul 24 '12 at 11:22
Think my confusion is because the Latin prefixes aren't used in months before September, and ages before Septuagenarian. And then eight is "Oct" for both Latin and Greek. – Urbycoz Jul 24 '12 at 11:32
Allegedly Hexadecimal was coined even though decimal is a latin root - because someone in 50s thought sexadecimal would be rude – mgb Jul 24 '12 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 '12 at 15:50

"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".

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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)

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