The "compositional threshold", in terms of Lin et al. [1], at around 60 mol % of GeCh(2) in (GeCh(2))y(P(2)Ch(3))(100-y) glasses (Ch = S, Se, P-pnictogen = Sb, Bi) where the local maximum of the glassforming ability was observed, is attributed to the formation of Ch(3)Ge-Ch-PCh(2) structural units. The Ch(3)Ge-Ch-PCh(2) structural units are suggested to be the basic units of the non-autonomous phase of eutectic (GeCh(2))y(P(2)Ch(3))(100-y) systems with the eutectic composition at around 60 mol. % of GeS2. (C) 2014 Elsevier B.V. All rights reserved.