There is no Proto-Indo-European language group. Proto-Indo-European, or PIE, is the hypothetical root language from which Indo-European languages today (and others that are extinct) descend.

Indo-European is believed to have originated from a hypothetical ancestral language called Proto-Indo-European, which is thought to have been spoken thousands of years ago on the Eurasian Steppe.

Proto-Indo-European languages are believed to have originated on the Pontic-Caspian steppe region, which covers parts of modern-day Ukraine and Russia. This location is known as the "homeland" or "Urheimat" of the Proto-Indo-Europeans based on linguistic and archaeological evidence. From this region, the Proto-Indo-European language spread and diversified into the various Indo-European languages we have today.

A. M. Lubotsky has written:

'The system of nominal accentuation in Sanskrit and proto-Indo-European' -- subject(s): Accents and accentuation, Indo-European languages, Nominals, Proto-Indo-European language, Word formation

'The System of Nominal Accentuation in Sanskrit and Proto-Indo-European (Memoirs of the Kern Institute No 4)'

Proto-Indo-European óynos