Where does “ἀν-” come from?

ἀν- (Ancient Greek) comes from Ancient Greek ὀρθός, from Proto-Hellenic ortʰwós, from Proto-Indo-European h₃r̥dʰwós, from Proto-Indo-European h₃erdʰ-, from Proto-Indo-European -wós — Creates adjectives from verb stems.

ἀν- (Ancient Greek): Alternative form of ἀ- used before a vowel;...