Allen and Greenough's New Latin Grammar, section 17

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Vowel Variations..

17.

The parent language showed great variation in the vowel sounds of kindred words.[1][This variation was not without regularity, but was confined within definite limits.]

a. This variation is often called by the German name Ablaut. It has left considerable traces in the forms of Latin words, appearing sometimes as a difference of quantity in the same vowel (as, u, ú; e, é), sometimes as a difference in the vowel itself (as, e, o; i, ae):[2][In Greek, however, it is more extensively preserved.] -

tegó, I cover, toga, a robe; pendó, I weigh, pondus, weight; fidés, faith, fídus, faithful, foedus, a treaty; miser, wretched, maestus, sad; dare, to give, dónum, a gift; regó, I rule, réx, a king; dux, a leader, dúcó (for older doucó), I lead. Compare English drive, drove (drave), driven; bind, bound, band; sing, sang, sung; etc.

Kindred Forms.

18. Both Latin and English have gone through a series of phonetic changes, different in the two languages, but following definite laws in each. Hence both preserve traces of the older speech in some features of the vowel system, and both show certain correspondences in consonants in words which each language has inherited from the old common stock. Only a few of these correspondences can be mentioned here.