Schenker A.M. The Proto-Indo-European obstruents in their functional aspect

Analecta Indoeuropaea Cracoviensia, vol. II: Kuryłowicz Memorial Volume. Part One. — Cracow: Universitas, 1995. — pp. 73-80.Among the Proto-Indo-European obstruents the opposition voiceless/voiced (tense/lax) was phonemically relevant with the unaspirated dental and velar stops only (t/d, k'/g', k/g, kʷ/gʷ) and was neutralized with other obstruents (P, Ph, Th, K'h, Kʷh, S and the laryngeals H1, H2, H3). Allophonic and typological patterning suggests that P was a stop but Ph, Th, K'h, Kh, Kʷh, S, H1, H2, H3 were fricatives.