In Indo-European studies, the term s-mobile (//; the word is a Latin neuter adjective) designates the phenomenon where a PIE root appears to begin with an *s- which is sometimes but not always present. It is therefore represented in the reflex of the root in some attested derivatives but not others.
This "movable" prefix s- appears at the beginning of some Indo-European roots, but is absent from other occurrences of the same root. For example, the stem *(s)táwros, perhaps 'bison', gives Latin taurus and Old English steor (Modern English steer), both meaning 'bull'. Both variants existed side by side in PIE, with Germanic preserving the forms as *steuraz and *þeuraz respectively, but Italic, Celtic, Slavic and others all have words for 'bull' which reflect the root without the s. Compare also: Gothic stiur, German Stier, Avestan staora (cattle); but Old Norse þjórr, Greek tauros, Latin taurus, Old Church Slavonic turъ, Lithuanian tauras, Welsh tarw, Old Irish tarb, Oscan turuf and Albanian taroç.
In other cases it is Germanic which preserves only the form without the s mobile. The root *(s)teg-, 'to cover', has descendants English thatch (Old English þeccan), German decken 'cover', Latin tegō 'cover', but Greek stégō and Russian stog. The fact that there is no consistency about which language groups retain the s-mobile in individual cases is good evidence that it is an original Indo-European phenomenon, and not an element added or lost in the later history of particular languages.
Sometimes subsequent developments can treat the forms with and without the s-mobile quite differently. For example, by Grimm's law PIE *p becomes Proto-Germanic f, but the combination *sp is unaffected by this. Thus the root *(s)prek, perhaps meaning 'scatter' has two apparently quite dissimilar derivatives in English: sprinkle (from nasalized form *sprenk-) and freckle (from *prek-).
S-mobile is always followed by another consonant. Typical combinations are with voiceless stops: *(s)p-, *(s)t-, *(s)k-; with liquids and nasals: *(s)l-, *(s)m-, *(s)n-; and rarely: *(s)w-.
One theory of the origin of the s-mobile is that it was influenced by a suffix to the preceding word; many inflectional suffixes in PIE are reconstructed as having ended in *s, including the nominative singular and accusative plural of nouns. The s-mobile can therefore be seen as an interference between the words, a kind of sandhi development. So for example, while an alternation between *péḱyont and *spéḱyont (both meaning 'they saw') might be difficult to imagine, an alternation between *wĺ̥kʷoms péḱyont and *wĺ̥kʷoms spéḱyont ('they saw the wolves' ) is plausible. The two variants would still be pronounced differently, as the double -ss- is distinct from a single -s- (compare English the sink and this sink), but the alternation can now be understood as a simple process of gemination (doubling) or degemination.
This can be understood in two ways.
|Root||Meaning||Reflexes with s-||Reflexes without s-|
|sk||*(s)kap-||tool||Ancient Greek skeparnion||Latin capus|
|*(s)kel-||crooked||German schielen 'squint', Greek skṓlēx 'worm'||Greek kō̃lon 'limb'|
|*(s)kep-||cut, scrape||English scab||Late Latin capulare 'cut'|
|*(s)ker-||cut||English shear, share, Polish skóra 'leather'||Latin curtus 'short', Russian korá 'cortex'|
|*(s)ker-||bent||English shrink, Avestan skarəna 'round'||Latin curvus 'curved', Russian kriv’ 'crooked', Lithuanian kreĩvas 'crooked'|
|*(s)kleu-||close||German schließen||Latin claudere|
|*(s)kʷal-o-||big fish||Latin squalus||English whale|
|sl||*(s)leug-||to swallow||German schlucken||Old Irish loingid 'eats', Ancient Greek lúzein 'hiccup', Polish łykać, połknąć 'swallow'|
|sm||*(s)melo-||small animal||English small||Dutch maal 'cow-calf', Irish míol 'animal', Russian mályj 'small'|
|*(s)meld-||melt||Dutch smelten||English melt, Ancient Greek méldein|
|sn||*(s)neh₂-||swim||Vedic Sanskrit snā́ti, Old Irish snáïd||Tocharian B nāskeṃ 'wash themselves'|
|*(s)nēg-o-||snake||English snake||Sanskrit nāga 'snake'|
|sp||*(s)peik-||woodpecker, magpie||German Specht 'woodpecker'||Latin pica 'magpie'|
|*(s)per-||sparrow||English sparrow, Ancient Greek psár 'starling', Polish szpak 'starling'||Latin parra|
|*(s)plei-||split||English split, splinter||English flint|
|*(s)poi-||foam||Latin spuma||English foam, Polish piana 'foam'|
|st||*(s)teh₂-||stand||Latin stare, English stand||Irish tá 'be'|
|*(s)twer-||whirl||English storm||Latin turba 'commotion'|
|*(s)ton-||thunder||Greek stenein||English thunder, Latin tonare|
|sw||*(s)wagʰ-||resound||English sough||Ancient Greek ēkhḗ 'sound'|
|*(s)wendʰ-||dwindle, wither||German schwinden 'dwindle'||Russian vjánut′, uvjadát′ 'wither', Polish więdnąć 'wither'|
A number of roots beginning in *sl-, *sm-, *sn- look as if they had an s-mobile but the evidence is inconclusive, since several languages (Latin, Greek, Albanian) lost initial s- before sonorants (l, m, n) by regular sound change. Examples include:
|Root||Meaning||Reflexes with s-||Reflexes without s-|
|sl||*(s)leg-||slack||English slack||Old Irish lacc, Ancient Greek lagarós|
|*(s)lei-||slimy||English slime, Irish sleamhuin ‘smooth’, Lithuanian sliẽnas, Polish ślimak ‘snail’||Latin limus ‘muck’, Ancient Greek leímax ‘snail’|
|sm||*(s)mek-||chin||Hittite zama(n)gur, Irish smeach, Old English smǣras ‘lips’, Lithuanian smãkras, smakrà, Sanskrit śmaśru||Latin māxilla, Albanian mjekër, Armenian mawrukʿ|
|sn||*(s)neigʷh-||snow||English snow, Latvian snìegs, Russian sneg, Avestan snaēža-, Sanskrit snéha||Latin nix, Ancient Greek nípha|
|*(s)nus-||daughter-in-law||Icelandic snör, Czech snacha, Persian suna, Sanskrit snuṣā́||Latin nurus, Ancient Greek nyós, Armenian nu|