The roots of the reconstructed Proto-Indo-European language (PIE) are basic parts of words to carry a lexical meaning, so-called morphemes.
[4] Verbal endings convey information about grammatical person, number and voice.
This can hold even for roots that are often translated as nouns: *ped-, for example, can mean 'to tread' or 'foot', depending on the ablaut grade and ending.
[11] In its base form, a PIE root consists of a single vowel, preceded and followed by consonants.
Except for a very few cases, the root is fully characterized by its consonants, while the vowel may change in accordance with inflection or word derivation.
Thus, the root *bʰer- can also appear as *bʰor-, with a long vowel as *bʰēr- or *bʰōr-, or even unsyllabic as *bʰr-, in different grammatical contexts.
The onset and coda must contain at least one consonant; a root may not begin or end with the ablaut vowel.
Such simple roots are common; examples are: *deh₃- 'to give', *bʰer- 'to bear', *dʰeh₁- 'to put', *dʰew- 'to run', *h₁ed- 'to eat', *h₂eḱ- 'sharp', *ped- 'to tread', *sed- 'to sit', and *wes- 'to clothe'.
These include: *dʰwes- 'to breathe', *h₁rewdʰ- 'red', *h₂erh₃- 'to plough', *h₃reǵ- 'straight', *leyǵ- 'to bind', *prews- 'to freeze', *srew- 'to flow', *swep- 'to sleep', and *wleykʷ- 'to moisten'.
[14] Early PIE scholars reconstructed a number of roots beginning or ending with a vowel.
[15] The latter type always had a long vowel (*dʰē- 'to put', *bʰwā- 'to grow', *dō- 'to give'), while this restriction did not hold for vowel-initial roots (*ed- 'to eat', *aǵ- 'to drive', *od- 'to smell').
[17] The rules for the ordering within a cluster of obstruents are somewhat different, and do not fit into the general sonority hierarchy: In several roots, a phenomenon called s-mobile occurs, where some descendants include a prepended *s while other forms lack it.
In particular, no examples are known of roots containing two plain voiced plosives (**ged-) or two glides (**ler-).
A few examples of roots with two fricatives or two nasals (*h₂eh₃- 'to burn', *nem- 'to give, to take', etc.)
An exception, however, were the voiced aspirated and voiceless plosives, which relatively commonly co-occurred (e.g. *dʰegʷʰ- 'to burn', *peth₂- 'to fly').
[14][19] An additional constraint prohibited roots containing both a voiced aspirated and a voiceless plosive (**tebʰ-), unless the latter occurs in a word-initial cluster after an *s (e.g. *stebʰ- 'to stiffen').
[14] Taken together with the abundance of *DʰeDʰ-type roots, it has been proposed that this distribution results from a limited process of voice assimilation in pre-PIE, where a voiceless stop was assimilated to a voiced aspirate, if another one followed or preceded within a root.
Such roots can be seen as generalized zero grades of unattested forms like **bʰweh₂-,[21] and thus follow the phonotactical rules.
*pster-, for example, might not have existed in PIE at all, if the Indo-European words usually traced back to it are onomatopoeias.
The characterised imperfective stems are often different in different descendants, but with no association between certain forms and the various branches of Indo-European, which suggests that a number of aspects fell together before PIE split up.
For example, the ablauting noun *h₂óy-u ~ *h₂y-éw- 'lifetime' was formed as a u-stem derivative of the root *h₂ey-.