From Paul Friedrich's article in EIEC, 'Numerals', under 'Ten'
(p.403), (diacritics omitted):
--start quote--
PIE *dekm may be from *de-kont-, de-kmt-, 'two units, two hands; unit
of ten'(< *kmtom 'unit', large unit; hundred'). At least as plausible
is a relation to *dek- 'take' from which *deks, an old word for 'hand'
or 'right hand' (cf Lat dexter 'right hand', etc.), if IE hand
counting started with the left hand and ended with the tenth finger on
the right hand.
--end quote--
Not so much 'twice five' as 'two sets if fingers and thumbs;
sinister+dexter'.
In the same article, under 'Hundred' (p.405) Friedrich says:
--start quote--
Perhaps *kmtom was originally '(numerical) unit', i.e., 'five' and
related to Gmc *handu- 'hand', and thus *de-kmt 'two units', i.e.,
'ten' and *(d)kmtom 'tenth decade', i.e., 'hundred'.
--end quote--
I find it more plausible that 100 was built up from something based on
five and/or ten.
Mark.
> Nor do I, to be honest. At any rate, "probably" is a gross
overstatment.
>
> Piotr
> Mark O:
> >The interesting thing is that the ten-word probably means
twice-five
> >(duo-dkm),[...]
>
> Say wha? Run that by me again. I don't see that being feasible.
>
> - gLeN