In Ancient Greek, why ἑπτά vs. ἕβδομος?

Question

I was marveling today at the word hebdomadal, from the Greek ἑπτά for seven. But that had me wondering why words derived from seven sometimes use /bd/ and other times /pt/. I notice, for instance, that the word for seventh is ἕβδομος. (The same could be asked of ὀκτώ and ὄγδοος.) And among compound numerals, the cardinals persist in using /pt/ and /kt/, while the ordinals persist in using /bd/ and /gd/. What's going on here?

I consulted Smyth's Greek Grammar, and it distinguishes in §16 between the order of stops—/p/ and /t/, as unvoiced stops, belong to the first order, while /b/ and /d/, as voiced stops, belong to the second—and per §82, a labial or palatal before a dental changes to the dental's order. Smyth lists ἑπτά as an example in §82.c. N. 1, so I think I'm on the right track. It appears the dental is changing from smooth to middle order, and the labial with it—my question is, why?

Answer 1

The Greek word for seven, hepta, like Sanskrit sapta, Latin septem, and others, points to an Indo-European *septm. In Greek *s becomes h, and the syllabic *m becomes a. In the word for “seventh”, IE *septm-o-, the m is a consonant (non-syllabic) and remains m in Greek, giving *heptmos, and then (with voicing of pt to bd before the voiced m) *hebdmos, and then (with insertion of a vowel to break up the cluster) hebdomos.