Vowel changes

A morphological oddity that is evident in all I-E languages is the alternation of the vowel e with the vowel o. This is not a sound change but a morphological marker of Indo-European: thus the Greek verb pherō “I carry” has an e in the stem pher-, but the related noun phoros “tribute” has an o (stem phor-): the same process in the same root can be seen in English bear versus burden.—A Brief History of Ancient Greek, page 9

<idle musing>
I didn't know that. Interesting, isn't it. I put the book on order from interlibrary loan (I read the first chapter on the link above).
</idle musing>