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 studies phase culminates in Franz Bopp's "Comparative Grammar" of 1833. The classical phase of Indo-European comparative linguistics leads from Bopp to August Schleicher's 1861 "Compendium" and up to Karl Brugmann's 5-volume "Grundriss" (outline of Indo-European languages) published from 1886–1893. Brugmann's Neogrammarian re-evaluation of the field and Ferdinand de Saussure's proposal of the concept of "consonantal schwa" (which later evolved into the laryngeal theory) may be considered the beginning of "contemporary" Indo-European studies. The Indo-European proto-language as described in the early 1900s in its main aspects is still accepted today, and the work done in the 20th century has been cleaning up