Editing Afroasiatic languages (section)

===Consonant incompatibility===
{| class="wikitable" style="float:right; font-size:smaller;"
|+ Examples of root consonant incompatibilities from Egyptian, after {{harvnb|Allen|2020a}}
|-
! consonant !! cannot occur with
|-
| p || b, f, m, h
|-
| r || ꜣ, b
|-
| ḫ || h, ḥ, ẖ, q, k, g, ṯ, ḏ
|-
| s || ḥ, z
|-
| t || ꜥ, z, q, g, d, ḏ
|}
Restrictions against the co-occurrence of certain, usually similar, consonants in verbal roots can be found in all Afroasiatic branches, though they are only weakly attested in Chadic and Omotic.{{sfn|Bender|1978|p=9-10}} The most widespread constraint is against two different [[labial consonant]]s (other than ''w'') occurring together in a root, a constraint which can be found in all branches but Omotic.{{sfn|Ehret|1995|p=77, 488}} Another widespread constraint is against two non-identical [[lateral consonant|lateral]] [[obstruent]]s, which can be found in Egyptian, Chadic, Semitic, and probably Cushitic.{{sfn|Ehret|1995|p=395}} Such rules do not always apply for nouns, numerals, or [[denominal verb]]s, and do not affect prefixes or suffixes added to the root.{{sfn|Greenberg|1950a|p=178}} Roots that may have contained sequences that were possible in Proto-Afroasiatic but are disallowed in the daughter languages are assumed to have undergone consonant [[dissimilation]] or [[assimilation (linguistics)|assimilation]].{{sfn|Edzard|1992|p=153-154}}{{sfn|Takács|1999|pp=323–332}}

A set of constraints, developed originally by Joseph Greenberg on the basis of Arabic, has been claimed to be typical for Afroasiatic languages.{{sfn|Vernet|2011|p=1}} Greenberg divided Semitic consonants into four types: "back consonants" ([[glottal consonant|glottal]], [[pharyngeal consonant|pharyngeal]], [[uvular consonant|uvular]], [[laryngeal consonant|laryngeal]], and [[velar consonant]]s), "front consonants" ([[dental consonant|dental]] or [[alveolar consonant]]s), [[liquid consonant]]s, and [[labial consonant]]s. He showed that, generally, any consonant from one of these groups could combine with consonants from any other group, but could not be used together with consonants from the same group.{{sfn|Greenberg|1950a|p=178}} Additionally, he showed that Proto-Semitic restricted a sequence of two identical consonants in the first and second position of the triliteral root.{{sfn|Greenberg|1950a|pp=167–168}} These rules also have a number of exceptions:
# velar consonants can occur with pharyngeals or laryngeals;{{sfn|Vernet|2011|p=7}}
# dental consonants can co-occur with [[sibilant]]s;{{sfn|Bender|1978|p=10}} However, there are no Proto-Semitic verbal roots with ''ḍ'' and a sibilant, and roots with ''d'' and a sibilant are uncommon. In all attested cases of a dental and a sibilant, the sibilant occurs in first position and the dental in second.{{sfn|Vernet|2011|p=7}}
Similar exceptions can be demonstrated for the other Afroasiatic branches that have these restrictions to their root formation.{{sfn|Bender|1978|p=10}} [[James Peter Allen|James P. Allen]] has demonstrated that slightly different rules apply to Egyptian: for instance, Egyptian allows two identical consonants in some roots, and disallows velars from occurring with pharyngeals.{{sfn|Allen|2020a|p=90-92}}