In Indo-European studies, the term s-mobile (pron.: //; the word is a Latin neuter adjective) designates the phenomenon where a PIE root begins 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.
 General description
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)tauro-, perhaps 'bison', gives Latin taurus and Old English steor (Modern English steer), both meaning 'bull'. Both variants existed side by side in PIE, but whereas Germanic (aside from North Germanic) has preserved the form with the s mobile, 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 forms without the s mobile. The root *(s)teg-, 'to cover', gives us 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 proves 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-). Another such pair is spring and frog, from *(s)preu, 'to jump'.
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. Since the nominative of Indo-European nouns often ended in *-s and it seems to have been an essentially SVO language[dubious ], it follows that verbs were frequently preceded by this phoneme. 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 *pekyont and *spekyont (both meaning 'they saw') might be difficult to imagine, an alternation between *wlkwoms pekyont and *wlkwoms spekyont ('they saw the wolves'  -here incidentally in OV order) 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.
- Gemination (s→ss): by this view, the form without the *s- is original. A habit of doubling at the join of the words causes a second -s- which is understood as part of the second word. This is a kind of assimilation. Obviously this could not happen to related forms which were used in different syntactic positions, and thus the original form without the s- survives elsewhere. This is the explanation given by Sihler.
- Degemination (ss→s): by this view, the form with the *s- is original. When it is adjacent to a noun suffix in -s, this produces a geminate. In rapid speech this is reduced to a single -s- which is understood to belong to the noun, leaving the verb without its initial sibilant. This explanation is more popular among linguists, for two reasons. Firstly, because a simplification of geminate ss is also observable elsewhere in the language (e.g. PIE *h1és-si → *h1ési: see Indo-European copula). And secondly because most PIE roots beginning with the clusters sp-, st-, etc. have variants without the s-, whereas there are very many roots beginning with a simple p-, t-, etc. which have no s-mobile equivalents. If the variants without the s- are original, we would be faced with the problem of explaining why the phenomenon was not more widespread.
 Further examples
|Root||Meaning||Reflexes with s-||Reflexes without s-|
|sk||*(s)kap-||tool||Greek skeparnion||Latin capus|
|*(s)kel-||crooked||German schielen ('to squint'), Greek skolex ('worm')||Greek kolon ('limb')|
|*(s)kep-||cut, scrape||English scab||Late Latin capulare ('cut')|
|*(s)ker-||cut||English shear, share, Russian шкура (škura, 'skin')||Latin curtus ('short'), Russian кора (kora, 'cortex')|
|*(s)ker-||bend||English shrink||Latin curvus ('curved')|
|*(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 ('eat')|
|sm||*(s)melo-||small animal||English small||Gaelic mial ('louse'), Russian малый (malyj, 'small'), Greek μῆλον (mēlon, 'small cattle')|
|*(s)meld-||melt||English smelt (from Low German or Dutch)||English melt, Greek meldein|
|sn||*(s)neh₂-||swim||Vedic Sanskrit snā́ti||Tocharian B nāskeṃ ('wash themselves')|
|sp||*(s)peik-||woodpecker, magpie||German Specht ('woodpecker')||Latin pica ('magpie')|
|*(s)plei-||split||English split, splinter||English flint|
|*(s)poi-||foam||Latin spuma||English foam|
|*(s)perg-||sparrow||Old English spearwa||Latin parra|
|st||*(s)teh₂-||stand||Latin sto, 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||Greek ēkhō ('echo')|
|*(s)wendʰ-||dwindle, wither||German schwinden ('dwindle')||Old Church Slavonic u-vęnǫti ('wither')|
A number of roots beginning in *sl-, *sm-, *sn- look as if they had an s-mobile but probably have not, 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||Latin laxus|
|*(s)lei-||slimy||English slime||Latin linere ('anoint')|
|sm||*(s)mek-||chin||Irish smeach||Latin maxilla|
|sn||*(s)neigʷh-||snow||English snow||Latin nix|
|*(s)nus-||daughter-in-law||Old High German snuor||Latin nurus|
- Example from Andrew L. Sihler, New Comparative Grammar of Greek and Latin, OUP 1995, p.169.
- Watkins, Calvert (2000). The American Heritage Dictionary of Indo-European Roots. Houghton Mifflin Harcourt. ISBN 0-618-08250-6.
- Rix, Helmut; Kümmel, Martin; et al. (2001). Lexikon der indogermanischen Verben (in German) (2 ed.). Dr. Ludwig Reichert Verlag. ISBN 3-89500-219-4. OCLC 47295102.
- Mark R.V. Southern, Sub-Grammatical Survival: Indo-European s-mobile and its Regeneration in Germanic, Journal of Indo-European Studies Monograph 34 (1999).
- Kenneth Shields (1996). "Indo-European s-mobile and Indo-European morphology" (PDF). Emérita. LXIV, 2: pp. 249–254.