Where does “ei” come from?

ei (Proto-Indo-European) comes from Proto-Indo-European h₁e-.

Ancestry of “ei”, step by step

ei traces back through two separate lines of ancestry.

via Proto-Indo-European h₁e-

StepLanguageWordMeaning
1Proto-Indo-Europeanh₁e-

via Proto-Indo-European h₁o-

StepLanguageWordMeaning
1Proto-Indo-Europeanh₁o-
Every word from Proto-Indo-European h₁e-