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Proto-Indo-European root |
The roots of the reconstructed Proto-Indo-European language (PIE) are basic morphemes carrying a lexical meaning. By addition of suffixes, they form stems, and by addition of endings, these form grammatically inflected words (nouns or verbs).
Except for a very few cases, the PIE root is fully characterized by its constituent consonants, while the vowel may alternate, a process called ablaut. The roots as a rule have a single syllabic core, and by ablaut may either be monosyllabic or unsyllabic.
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The centre of a PIE root is the ablauting vowel (usually *e, perhaps sometimes *a1 in the full grade). This vowel constitutes a sonority peak that is preceded and followed by a sequence of consonants with progressively decreasing sonority values (i.e., the sonority has to fall toward both edges of the root). The sonority hierarchy is as follows:2
This gives the following root structure (with P being a plosive and
an empty position):

Note that *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.3
Other possible roots include *ped- "foot", *dʰwer- "door" and *wleikʷ- "to moisten". Forbidden are structures like **mter- (wrong order of phonemes) and **wmek- (two phonemes of the same group).
The remaining sounds, namely the laryngeals *h₁ *h₂ *h₃ and the sibilant *s, can occupy almost any place in the hierarchy.2 *s is particularly common in initial position (see s-mobile).4
Examples of such roots are *peth₂- "to fly", *treh₁w- "to nourish" and *streig- "to stroke".
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-inital cluster together with an s-mobile (e.g. *stebʰ- "to stiffen").4
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 *strenk- "tight").4
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", *ant- "front", *od- "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-, *h₂ent-, *h₃ed-, resulting in a short vowel). These reconstructions obey the mentioned rules.
It should be noted that some roots like *pster- "to sneeze" do not seem to follow these rules.2 This might be due to incomplete understanding of PIE phonotactics or to wrong reconstructions.
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
The source of these extensions is not known.4
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