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2.8.1. Regarding Consonant Change, different reconstructions might appear, too; as, for ghortos, garden, enclosure, later town (cf. Gmc. gardan, Lat. hortus, Gk. khortos, Phry. -gordum, O.Ir. gort, Lith. gardas, O.C.S. gradu, Alb. garth, etc.), some would reconstruct an alternative *ghordhos, so that both forms (in -t- and -dh-) fit perfectly into the schemes of dialectal phonological laws.
2.8.2. The so called s-Mobile (mobile pronounced as in Latin, it is a neuter adjective) refers to the phenomenon of alternating word pairs, with and without s before initial consonants, in stems with similar or identical meaning. This “moveable” prefix s- 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-.
NOTE. Examples include (s)ten-, compare O.Ind. stánati, Gk. sténō, O.Eng. stenan, Lith. stenù, O.Sla. stenjo, and without s- in O.Ind. tányati, Gk. Eol. ténnei, Lat. tonare, O.H.G. donar, Cel. Tanaros (name of a river). For (s)pek-, cf. O.Ind. spáśati, Av. spašta, Gk. skopós (<spokós), Lat. spektus, O.H.G. spehon, without s- in O.Ind. páśyati, Alb. pashë. For (s)ker-, cf. O.Ind. ava-, apa-skara-, Gk. skéraphos, O.Ir. scar(a)im, O.N. skera, Lith. skiriù, Illyr. Scardus, Alb. hurdhë (<*skṛd-), without s- in O.Ind. kṛnáti, Av. kərəntaiti, Gk. keíro, Arm. kcorem, Alb. kjëth, Lat. caro, O.Ir. cert, O.N. horund, Lith. kkarnà, O.Sla. korŭcŭ, Hitt. kartai-, and so on.
Such pairs with and without s are found even within the same dialect, as Gk. (s)tégos, “roof”, (s)mikrós, “little”, O.Ind. (s)tṛ, “star”, and so on.
NOTE. Some scholars believe it was a prefix in PIE (which would have had a causative value), while others maintain that it is probably caused by assimilations of similar stems – some of them beginning with an s-, and some of them without it. It is possible, however, that the original stem actually had an initial s, and that it was lost by analogy in some situations, because of phonetic changes, probably due to some word compounds where the last -s of the first word assimilated to the first s- of the second one. That helps to explain why both stems (with and without s) are recorded in some languages, and why no regular evolution pattern may be ascertained: so for example in wḷqons spekjont, they saw wolves, becoming wḷqons ‘pekjont. See Adrados (1995).
2.8.3. Before a voiced or aspirated voiced consonant, s was articulated as voiced, by way of assimilation; as, nisdos [‘niz-dos], nest, misdhom [‘miz-dhom], meed, salary, or osdos [‘oz-dos], branch. When s forms a group with sonants there is usually assimilation, but such a trend was sometimes reversed by adding a consonant; as Lat. cerebrum (<Ita. kereθrom), from kersrom [‘kerz-rom], brain.
NOTE. Related to the later assimilation of [s] into [z] between vowels, they became very unstable in some IE dialects, showing sometimes rhotacism; as, snusós, daughter-in-law, cf. Lat. nurus, O.H.G. snur; or genos, race, stock, kind, cf. Lat. genus, generis (<*geneses).
2.8.4. Similarly, the manner of articulation of an occlusive usually depends on its environment. Thus, voiced stops turn voiceless in final position; as, pods, foot, gives voiceless O.Ind. pāt, qid gives O.Ind. cit, agtós gives voiceless Gk. ακτος (aktos) or Lat. actus. The same happens with voiced aspirates, as in legh-, lie (cognate to Eng. log), giving Gk. λεκτρον (lektron), Lat. lectus, O.H.G. Lehter. Voiceless occlusives become voiced before voiced consonants; as, zero-grade ped- in Gk. επιβδα (epi-bd-a).
2.8.5. A sequence of two dentals, such as -tt-, -dt-, -tdh-, -ddh-, etc. was eliminated in all Indo-European dialects, but the process of this suppression differed among branches; Vedic Sanskritshowing little change, some others an intermediate -sT-, and others -ss- or -s-. Compounds were not affected by this trend; as, kréd-dhēmi, believe.
NOTE. This trend began probably in Late PIE, and thus all IE speakers knew such evolutions, which we sum up into a common intermediate stage -st-, -sdh-, etc., which was followed in some early IE dialects, and probably known to the rest of them. See the section Conventions Used in this Book for more on this question. For phonetic changes in Aryan dialects, see Appendix II.
Examples in MIE are e.g. forms derived from PIE root weid-, know, see, which gave verb widējō, cf. Lat. vidēre, Goth. witan, O.C.S. viděti, Lith. pavydéti; p.p. wistós, seen, from wid-tó-, (cf. O.Ind. vitta-, but Av. vista-, O.Pruss. waist, O.Sla. věstъ, or Gmc. wīssaz, Lat. vīsus, O.Gk. ϝιστος, O.Ir. rofess, etc.); noun wistis, sight, vision, from wid-ti-, cf. Goth wizzi, Lat. vīsiō; Greek wistōr, wise, learned man, from wid-tor, cf. Gk. ἵστωρ<*ϝίστωρ (wístōr), PGk wistorjā, history, from Gk. ἱστορία (historía); Imp. wisdhi! know!, from wid-dhí, cf. O.Ind. viddhí, O.Gk. ϝίσθι, O.Lith. veizdi, and so on.