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 "eat" or "run". Roots never occur alone in the language. Complete inflected words like verbs, nouns or adjectives are formed by adding further morphemes to a root. Typically, a root plus a suffix forms a stem, and adding an ending forms a word.
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 alternate, a process called ablaut. Thus, the mentioned root *bʰer- can also appear as *bʰor-, with a long vowel as *bʰēr- or *bʰōr-, or even unsyllabic as *bʰr-, in different grammatical contexts.
Basic root structure 
The centre of a PIE root is the ablauting vowel (usually *e, perhaps sometimes *a in its base form, the full grade). This vowel constitutes a sonority peak that is preceded and followed by a sequence of consonants with progressively decreasing sonority values. In other words, the sonority has to fall toward both edges of the root. The sonority hierarchy is as follows:
- *l *r *y *n
- *w *m
- plosives (sounds like *p *t *ḱ *k *kʷ or *bʰ; see Proto-Indo-European phonology for a complete table of PIE plosives)
This gives the following root structure (with P being any plosive and an empty position):
*w after a vowel is often written *u, and *y after a vowel is often written *i. Thus, *leiǵ- = *leyǵ- "to bind" and *dʰeu- = *dʰew- "to run" are allowed roots.
Other possible roots include *ped- "to tread", *dʰwes- "to breathe" and *wleikʷ- "to moisten". Forbidden are structures like **mter- (wrong order of phonemes: internal plosive) and **wmek- (two phonemes of the same group: unchanging sonority).
Additional phonemes 
The remaining sounds, namely the laryngeals *h₁ *h₂ *h₃ and the sibilant *s, can occupy almost any place in the hierarchy. *s is particularly common in initial position (see s-mobile). Examples of such roots are *peth₂- "to fly", *treh₁w- "to nourish" and *streig- "to stroke".
Restrictions on the plosives 
A root cannot contain two plain voiced plosives (**ged-), nor can it contain a voiced aspirate and a voiceless plosive (**tebʰ-), unless the latter occurs in a word-initial cluster after an *s (e.g. *stebʰ- "to stiffen").
Restrictions on the number of phonemes 
The vowel has to be preceded and followed by at least one consonant each. The maximum number of consonants seems to be five (as in *strengʰ- "to twine").
Early PIE scholars reconstructed a number of roots beginning or ending with a vowel. 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"). Laryngeal theory can explain this behaviour by reconstructing a laryngeal following the vowel (*dʰeh₁-, *bʰweh₂-, *deh₃-, resulting in a long vowel) or preceding it (*h₁ed-, *h₂eǵ-, *h₃ed-, resulting in a short vowel). These reconstructions obey the mentioned rules.
Roots without a full grade 
Some roots like *pster- "to sneeze" or *pteh₂k- "to duck" do not appear to follow these rules. This might be due to incomplete understanding of PIE phonotactics or to wrong reconstructions. *pster-, for example, might not have existed in PIE at all, if the Indo-European words usually traced back to it are onomatopoeias.
Thorn clusters are sequences of a dental (*t *d *dʰ) plus a velar plosive (*k *g *gʰ etc.). Their role in PIE phonotactics is unknown. Roots like *dʰgʷʰei- "to perish" apparently violate the phonotactical rules, but are quite common.
Lexical meaning 
The meaning of a reconstructed root is conventionally that of a verb; the terms root and verbal root are almost synonymous in PIE grammar. This is because, apart from a limited number of so-called root nouns, PIE roots overwhelmingly participate in verbal inflection through well-established morphological and phonological mechanisms. Their meanings are not always directly reconstructible, due to semantic shifts that led to discrepancies in the meanings of reflexes in the attested daughter languages. Many nouns and adjectives are derived from verbal roots via suffixes and ablaut.
Nevertheless, some roots did exist that did not have a primary verbal derivation. Apart from the aforementioned root nouns, the most important of these were the so-called Caland roots, which had adjectival meaning. Such roots generally formed proterokinetic adjectives with the suffix *-u-, thematic adjectives in *-ró- and compounding stems in *-i-. They included at least *h₁rewdʰ- "red", *h₂erǵ- "white", *dʰewb- "deep" and *gʷreh₂- "heavy".
Word formation 
Fully inflected words are usually formed from a root plus a suffix plus an ending. The suffix is sometimes missing, which has been interpreted as a zero suffix. Words with zero suffix are termed root verbs and root nouns. Beyond this basic structure, there is the nasal infix, a present tense marker, and reduplication, a sort of prefix with a number of grammatical and derivational functions.
Finite verbs 
Verbal suffixes, including the zero suffix, convey grammatical information about tense and aspect, two grammatical categories that are not clearly distinguished. Present and aorist are universally recognised, while some of the other aspects remain controversial. Two of the four moods, the subjunctive and the optative, are also formed with suffixes, which sometimes results in forms with two consecutive suffixes: *bʰér-e-e-ti > *bʰérēti "he would carry", with the first *e being the present tense marker, and the second the subjunctive marker. Reduplication can mark the present and the perfect.
Nouns and adjectives 
Nouns are usually derived from roots or verb stems by suffixation or other means (see the morphology of the Proto-Indo-European noun for some examples). 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. Some nouns like *agʷn-o- "lamb" or *h₂ster- "star", however, are not derived from verbal roots. In any case, the meaning of a noun is given by its stem, whether this is composed of a root plus a suffix or not. This leaves the ending, which conveys case and number.
Adjectives are also derived by suffixation of (usually verbal) roots. An example is *ǵn̥h₁-tó-s "begotten, produced" from the root *ǵenh₁- "to beget, to produce". The endings are the same as with nouns.
Infinitives and participles 
Infinitives are verbal nouns and, just like other nouns, are formed with suffixes. It is not clear whether any of the infinitive suffixes reconstructed from the daughter languages (*-dʰje-, *-tu-, *-ti-, among others) was actually used to express an infinitive in PIE.
Participles are verbal adjectives formed with the suffixes *-ent- (active imperfective and aorist participle), *-wos- (perfect participle) and *-mh₁no- or *-m(e)no- (mediopassive participle), among others.
Root extensions 
Root extensions are additions of one or two sounds, often plosives, to the end of a root which do not seem to change its meaning. For *(s)teu- "to push, hit, thrust", we can reconstruct
- *(s)teu-k- > Ancient Greek τύκος (túkos) "hammer"
- *(s)teu-g- > English stoke (Germanic k goes back to PIE *g.)
- *(s)teu-d- > Vedic tudáti "beats"
The source of these extensions is not known.
See also 
- Lexikon der indogermanischen Verben ("Lexicon of the Indo-European Verbs", in German), a lexicon of PIE verbal roots
- Fortson (2004:76)
- The asterisk * indicates that this form is not directly attested, but has been reconstructed on the basis of other linguistic material.
- All examples of PIE roots are taken from Rix (2001) and Fortson (2004).
- The existence of *a as an ablauting vowel is disputed (see Indo-European ablaut: a-grade).
- Rix (2001:5)
- Fortson (2004:70–73)
- Pokorny (1959)
- Meier-Brügger, Fritz & Mayrhofer (2003, L 321)
- Rix (2001:98–99)
- Jasanoff (2003:112)
- Mallory & Adams (1997:133)
- Fortson (2004:59–60)
- Ringe (2006)
- Fortson (2004:108)
- Rix (2001:14–21)
- Fortson (2004:81–83)
- Fortson (2004:83–85)
- Fortson (2004:116, 302)
- Fortson (2004:103)
- Fortson (2004:120–121)
- Fortson (2004:97)
- Fortson (2004:97–98)
- Brugmann, Karl; Delbrück, Berthold (1886). Grundriß der vergleichenden Grammatik der indogermanischen Sprachen.
- Buck, Carl Darling (15 June 1988). A dictionary of selected synonyms in the principal Indo-European languages: A contribution to the history of ideas (Reprint edition). University of Chicago Press. ISBN 0-226-07937-6.
- Fortson, Benjamin W., IV (2004). Indo-European Language and Culture. Blackwell Publishing. ISBN 1-4051-0316-7.
- Jasanoff, Jay (2003). Hittite and the Indo-European Verb. Oxford University Press. ISBN 0-19-928198-X.
- Köbler, Gerhard (1980). Indogermanisches Wörterbuch [Indo-European Dictionary] (in German).
- Mallory, J. P.; Adams, D. Q. (1997). Encyclopedia of Indo-European Culture. Routledge. ISBN 1-884964-98-2.
- Meier-Brügger, Michael; Fritz, Matthias; Mayrhofer, Manfred (2003). Indo-European Linguistics. Berlin; New York: Walter de Gruyter. ISBN 3-11-017433-2.
- Pokorny, Julius (1959). Indogermanisches etymologisches Wörterbuch. French & European Publications. ISBN 0-8288-6602-3.
- Ringe, Don (2006). A Linguistic History of English part 1: From Proto-Indo-European to Proto-Germanic.
- Rix, Helmut (2001). Lexikon der indogermanischen Verben. Dr. Ludwig Reichert Verlag. ISBN 3-89500-219-4.
- Watkins, Calvert (14 September 2000). The American Heritage Dictionary of Indo-European roots: Second Edition. Houghton Mifflin. ISBN 0-395-98610-9.
|Look up Appendix:List of Proto-Indo-European roots in Wiktionary, the free dictionary.|
- American Heritage Indo-European Roots Index
- Database query to the online version of Pokorny's PIE dictionary
- Index to the online version of Pokorny's PIE dictionary
- Jonathan Slocum, Indo-European Lexicon from the University of Texas Linguistic Research Center