The core idea of this root was:
And there are basically four big branches we can identify. The first one is duo, and here the sense of two is pretty obvious.
- doubt (“on two sides)”
- dubious (“two faced”)
- duplicity (“two faced”)
The next branch is around the syllables: di-, dia-. In some words in this branch the idea of two is pretty clear. Others are about a theme of through which is probably an extension of the idea of from one side to the other.
- carbon dioxide (“carbon with two oxygen”)
- diploma (originally “official paper folded twice”)
- diabetes (“passing through”, referred to excessive peeing)
- diarrhea (“flowing through”, referred to.. well… you know)
- diagram (“marked/bordered by lines”)
The third branch is bi-.
Here, the theme of two is also very strong, but sometimes it’s fairly obscured.
- binocular (“two eyes”)
- biscuit (“cooked twice”)
- combine (“make two, put together”)
- balance (originally “scale with two pans”)
Last but not least, we have the Germanic branch of two, twi- and zw(e)i.
Besides the actual numbers, this branch has a quite a few words that are about coming from two sides.
- two, zwei, twelve, zwölf, twenty, zwanzig
- twilight (“double light”)
- between (“by/within two”)
- twine, twist (probably originally about “twisting two threads together for a rope”)
- twig, Zweig (branch splitting up “two ways”)
- Zweifel (“on two sides”)
- zwicken (to pinch – “pinch between two”)
- zwingen, Zwang (to force – “pinch between two”)
The connection of “Zweck” was an assumption in etymological sources for a while, but it’s not supported by newer research. Still, I decided to include it here.