The new reconstruction of Proto-Basque offered in Part I results in Proto-Basque roots, stems, and words that bear a striking resemblance to Proto-Indo-European reconstructions. Application of the comparative method yields regular sound correspondences between Proto-Basque and Proto-Indo-European vowels and consonants as detailed in this chapter and supported by lexical comparisons in the appendix. Some of the more striking correspondence are correspondences between PIE laryngeals and Proto-Basque *h, correspondences between PIE root-final palatalized velars and Proto-Basque *Ki sequences, correspondences between PIE root-final labialized velars and Proto-Basque *Ku sequences, and correspondences between PIE breathy-voiced consonants and Proto-Basque *DVh sequences. It is argued that regular sound correspondences between Proto-Basque and Proto-Indo-European in proposed cognates, including basic vocabulary and grammatical morphemes, are not accidental and are not due to borrowing. These correspondences may therefore be taken as evidence of common descent. Conservative phonological and morphological features of Proto-Basque in relation to Proto-Indo-European suggest that Proto-Basque did not descend from Proto-Indo-European. Instead, Proto-Basque and Proto-Indo-European should be seen as sister languages, or daughters of sister languages, descending from the same mother tongue.