2/Linguistic Note

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Linguistic Note on $2$

There are many words in natural language which indicate a twofold collection:

dual, duel, brace, couple, pair, double and twin

for example.


An archaic term for an instance of the number two is dyad.

This can be found in Sir Thomas L. Heath's $1908$ translation of Euclid: The Thirteen Books of The Elements, in Power of Two is Even-Times Even Only, but never specifically defined there formally.

In the words of Euclid:

Each of the numbers which are continually doubled beginning from a dyad is even-times even only.

(The Elements: Book $\text{IX}$: Proposition $32$)


Sources