the roots of the reconstructed proto-indo-european language (pie) are basic parts of words that carry a lexical meaning, so-called morphemes. pie roots usually have verbal meaning like "to eat" or "to run". roots never occur alone in the language. complete inflected verbs, but also nouns and adjectives are formed by adding further morphemes to a root and by changing the root's vowel in a process called ablaut.
a root consists of a central vowel that is preceded and followed by at least one consonant each. a number of rules have been determined that specify which consonants can occur together, and in which order. basically, the consonants with the highest sonority (*l, *r, *y, *n) are nearest to the vowel, and the ones with the lowest sonority such as plosives are furthest away, notwithstanding certain exceptions such as thorn clusters.
sometimes new roots were created in pie or its early descendants by various processes such as root extensions (adding a sound to the end of an existing root) or metathesis.