A-grade
It is still a matter of debate whether PIE had an original a-vowel at all. In later PIE, the disappearance of the laryngeal h2 could leave an a-colouring and this may explain all occurrences of a in later PIE. However some argue that the e-grade could sometimes be replaced by an a-grade without the influence of a laryngeal. This is controversial, but might help to explain the vowels in class 6 Germanic verbs, for example.
Read more about this topic: Indo-European Ablaut