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
Proto-Indo-European verbs of affixes, another way that vowels in the word could be modified was the Indo-European ablaut, which is still visible in languages such as the Germanic languages. For example, the vowel in the English (a Germanic language) verb "to sing" varies according to the conjugation of the verb: "sing", "sang" and "sung". The system described here is known as the "Cowgill–Rix" system and, strictly speaking, applies only to what Don Ringe terms "Western Indo-European" (Western IE), i.e. IE excluding Tocharian and especially Anatolian. The system also describes Tocharian fairly well, but encounters significant difficulties when applied to Hittite and the