In Indo-European studies, the term s-mobile designates the phenomenon where a PIE root appears to begin with an which is sometimes but not always present. It is therefore represented in the reflex of the root in some attested derivatives but not others. 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 any specific language.
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 'large domestic animal, cattle', perhaps 'aurochs', gives Latin Latin: taurus and Old English English, Old (ca.450-1100);: steor (Modern English 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 having words for 'bull' which reflect the root without the *s. Compare also: Gothic Gothic: stiur, German German: Stier, Avestan Avestan: staora (cattle); but Old Norse Norse, Old: þjórr, Greek Greek, Ancient (to 1453);: tauros, Latin Latin: taurus, Old Church Slavonic Church Slavic; Old Slavonic; Church Slavonic; Old Bulgarian; Old Church Slavonic: turъ, Lithuanian Lithuanian: tauras, Welsh Welsh: tarw, Old Irish Irish, Old (to 900);: tarb, Oscan turuf, and Albanian Albanian: taroç.
In other cases, it is Germanic that preserves only the form without the s mobile. The Proto-Indo-European root , 'to cover', has descendants English English: thatch (from Old English English, Old (ca.450-1100);: þeccan), German German: decken 'to cover', Latin Latin: tegō 'I cover', but Greek Greek, Ancient (to 1453);: stégō and Russian Russian: stog.
Sometimes subsequent developments can treat the forms with and without the s-mobile quite differently. For example, by Grimm's law PIE becomes Proto-Germanic *f, but the combination is unaffected by this. Thus the root , perhaps meaning 'to scatter', has two apparently quite dissimilar derivatives in English: English: sprinkle (from the nasalized form ), and English: freckle (from ).
S-mobile is always followed by another consonant. Typical combinations are with voiceless stops: , , ; with liquids and nasals: , , ; and rarely, .
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 many nouns. The s-mobile can therefore be seen as an interference between the words, a kind of sandhi or rebracketing development. So for example, while an alternation between and (both meaning 'they saw') might be difficult to imagine, an alternation between and ('they saw the wolves'[1]) is plausible. The two variants would still be pronounced differently, as the double -ss- is distinct from a single -s- (compare English this pot and this spot), but the alternation can now be understood as a simple process of gemination (doubling) or degemination.
This can be understood in two ways.
| Root[2] [3] | Meaning | Reflexes with s- | Reflexes without s- | |
| sk | to shine, glow | Sanskrit -ścandrá | Sanskrit candrá, Ancient Greek kándaros, Latin candeō, Old Armenian xand | |
| tool | Ancient Greek skeparnion | Latin capus | ||
| crooked | German schielen 'squint', Greek skṓlēx 'worm' | Greek kō̃lon 'limb' | ||
| cut, scrape | English scab | Late Latin capulare 'cut' | ||
| cut | English shear, share, Polish skóra 'leather' | Latin curtus 'short', Polish kora 'cortex' | ||
| bent | English shrink, Avestan skarəna 'round' | Latin curvus 'curved', Russian kriv’ 'crooked', Lithuanian kreĩvas 'crooked' | ||
| close (verb) | German schließen | Latin claudere | ||
| big fish | Latin squalus | English whale | ||
| sl | to swallow | German schlucken | Old Irish loingid 'eats', Ancient Greek lúzein 'hiccup', Polish łykać, połknąć 'swallow' | |
| sm | small animal | English small | Irish míol 'animal', Russian mályj 'small', Dutch maal 'calf (animal)', | |
| melt | English smelt, Dutch smelten | English melt, Ancient Greek méldein | ||
| sn | swim | Vedic Sanskrit snā́ti, Old Irish snáïd | Tocharian B nāskeṃ 'wash themselves' | |
| snake | English snake | Sanskrit nāga 'snake' | ||
| sp | woodpecker, magpie | German Specht 'woodpecker' | Latin pica 'magpie' | |
| sparrow | English sparrow, Ancient Greek psár 'starling', Polish szpak 'starling' | Latin parra | ||
| split | English split, splinter | English flint | ||
| foam | Latin spuma | English foam, Polish piana 'foam' | ||
| st | stand | Latin stare, English stand | Irish tá 'be' | |
| whirl | English storm | Latin turba 'commotion' | ||
| thunder | Greek stenein | English thunder, Latin tonare | ||
| sw | resound | English sough | Ancient Greek ēkhḗ 'sound' | |
| dwindle, wither | German schwinden 'dwindle' | Russian vjánut′, uvjadát′ 'wither', Polish więdnąć 'wither' |
A number of roots beginning in 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 | slack (weak, loose) | English slack, Old Norse slakr | Old Irish lacc, Ancient Greek lagarós | |
| slimy | English slime, Irish sleamhuin ‘smooth’, Lithuanian sliẽnas, Polish ślimak ‘snail’ | Latin limus ‘muck’, Ancient Greek leímax ‘snail’ | ||
| sm | chin | Hittite zama(n)gur, Irish smig, Old English smǣras ‘lips’, Lithuanian smãkras, smakrà, Sanskrit śmaśru | Latin māxilla, Albanian mjekër, Armenian mawrukʿ | |
| sn | snow | English snow, Latvian snìegs, Russian sneg, Avestan snaēža-, Sanskrit snéha | Latin nix, Ancient Greek nípha, Welsh nyf | |
| daughter-in-law | German Schnur,[4] Icelandic snör, Czech snacha, Persian suna, Sanskrit snuṣā́ | Latin nurus, Ancient Greek nyós, Armenian nu | ||