Classes of phonemes

There are four groups (classes) of consonantal phonemes in Georgian. Phonemes belonging to the one and the same class usually do not follow each other (do not form a consonant cluster) within one morpheme at the beginning of a word in Georgian:
I (class C1): b p p'  m v (labials)
II (class C2): d t t'  dz c c'  z  s  dzh ch  ch'  zh  sh  (frontlingual noises)
III (class C3): r l n (frontlingual sonorous)
IV (class C4): g k k'  g x q' (backlinguals), h (laryngeal)
(Tevdore Uturgaidze, kartuli enis ponematuri st'rukt'ura, The phonemic structure of the Georgian language, Tbilisi, Mecniereba, 1976).