I'm studying the secondary and double articulation, but got confused. Is double and primary articulation the same thing or what is the difference?
Secondary articulation refers to a "lesser" constriction, compared to the constriction of primary place of articulation. So in [tʷ, sʷ], the primary place of articulation is alveolar, and there is a secondary (vocalic-ish) labial articulation. Typically, secondary articulations are glide-like. "Double articulation" refers to segments which have two places of constriction which as "equal" in terms of constriction, for example [kp]. The phonological analysis of double articulations is controversial (the majority of evidence indicates that one of the places is indeed primary, but languages can differ in their analysis so that [kp] may behave like a "very labialized velar", or a "very velarized labial". Also note that these terms are quite variable in their use, so that [kʷ] may be called a "double articulation".