Indo-European copula copula (with corresponding voiceless and voiced fricatives) existed side by side in Germanic, and the involvement of a separate root "" is unnecessary. The root "*(s)teh-" meant "to stand". From this root comes the present stem of the so-called "substantive verb" in Irish and Scottish Gaelic, "tá" and "tha" respectively. On the absence of the initial s- in Celtic, see Indo-European s-mobile. In Latin, "stō, stare" retained the meaning "to stand", until local forms of Vulgar Latin began to use it as a copula in certain circumstances. Today, this survives in that several Romance languages (Galician-Portuguese, Spanish, Catalan) use it
Indo-European s-mobile 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 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 and (both meaning 'they saw') might be difficult to imagine, an