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Co-articulated consonants or complex consonants are consonants produced with two simultaneous places of articulation. They may be divided into two classes: doubly articulated consonants with two primary places of articulation of the same manner (both stop, or both nasal, etc.), and consonants with secondary articulation, that is, a second articulation not of the same manner.
An example of a consonant with secondary articulation is the voiceless labialized velar stop [k?] has only a single stop articulation, velar [k], with a simultaneous approximant-like rounding of the lips.
There is a large number of common secondary articulations. The most frequently encountered are labialization (such as [k?]), palatalization (such as the Russian "soft" consonants like [p?]), velarization (such as the English "dark" el [l?]), and pharyngealization (such as the Arabic emphatic consonants like [t?]).
As might be expected from the approximant-like nature of secondary articulation, it is not always easy to tell whether a co-articulated approximant consonant such as /w/ is doubly or secondarily articulated. In some English dialects, for example, /w/ is a labialized velar that could be transcribed as , but the Japanese /w/ is closer to a true labial-velar, [?]. It is common usage to restrict the letter ⟨w⟩ to the former.
The glottis controls phonation, and works simultaneously with many consonants. It is not normally considered an articulator, and an ejective such as [k'], with simultaneous closure of the velum and glottis, is not normally considered to be a co-articulated consonant.