Consonant letter d used alone means "two root letters".
From an English point of view "two root letters" would better be called "t". But then there would have been a confusion with the concept "three" root letters.
So we decided to take the first letter of the Latin numeral "duo", which means "two".
In fact, many are the languages which have letter d as the first letter of the word they use for number two, 2: French (deux), Italian (due), Portuguese (dois), Spanish and Catalan (dos)...
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