Research into reconstruction of the consonants of Middle Mongol has engendered several controversies. Middle Mongol had two series of plosives, but there is disagreement as to which phonological dimension they lie on, whether aspiration or voicing. The early scripts have distinct letters for velar plosives and uvular plosives, but as these are in complementary distribution according to vowel harmony class, only two back plosive phonemes, *"/k/", *' (~ *', *') are to be reconstructed. One prominent, long-running disagreement concerns certain correspondences of word medial consonants among the four major scripts ("UM", "SM", "AM", and "Ph", which were discussed in the preceding section). Word-medial "/k/" of Uyghur Mongolian (UM) has not one, but two correspondences with the three other scripts: either /k/ or zero. Traditional scholarship has reconstructed *"/k/" for both correspondences, arguing that *"/k/" was lost in some instances, which raises the question of what the conditioning factors of those instances were. More recently, the other possibility has been assumed; namely, that the correspondence between UM "/k/" and zero in the other scripts points to a distinct phoneme, "/h/", which would correspond to the word-initial phoneme "/h/" that is present in those other scripts. "/h/" (also called "/x/") is sometimes assumed to derive from *', which would also explain zero in "SM", "AM", "Ph" in some instances where "UM" indicates /p/; e.g., "debel" > Khalkha "deel".