old chinese probably had some constraint governing the shape of disyllables. according to modern reconstructions of old chinese phonology, type a and b syllables almost never co-occur in a disyllabic word. in the latest reconstruction of old chinese phonology proposed by baxter and sagart (2014), this type a vs. type b distinction can be traced back to the presence or the absence of pharyngealization respectively, cf. < oc "to bring into" (type a) and < oc "to enter" (type b) only differing by the [±pharyngeal] trait of the initial consonant. onsets of type b syllables, lacking of pharyngealization, are subject to palatalization in middle chinese (indicated by a palatal medial "-j-" in baxter's notation), while type a pharyngealized onsets failed to palatalize. in many ancient disyllabic words type a and type b characters do not mix, there are almost solely bisyllabic morphemes either with type a syllables, such as: