Bluetooth backwards compatibility which way?

[Dicky-Art]

OK I'm missing the obvious here!!

Backwards compatibility but from what to what?

V2.1 unit would pair to V3 earphones? Or
V3 unit pair to V2.1 earphones?

Thanks for any help

 

[GeorgeHillier]

I believe it works either way, for example bluetooth V3 and V4 devices connect fine. (Correct me if I'm wrong)