Multiple articulations

It is possible to have more than one constriction gesture, that is, it's possible to narrow the vocal tract at two or more places at the same time. Some examples we have seen of this in English are:

[w]: a simultaneous dorso-velar approximant and bilabial approximant (or lip rounding)
dark [ɫ]: a simultaneous apico-alveolar lateral approximant and dorso-velar central approximant
[ɹ]: simultanous postalveolar approximant, pharyngeal approximant, and lip rounding.

While nasality and the state of the glottis are properties of the entire consonant, we have to answer four of the questions separately for each constriction:

active articulator
passive articulator
constriction degree
laterality

Multiple articulations are often classified as double articulations and secondary articulations on the basis of whether the two constrictions are equal in degree.

Double articulations

Double articulations are those cases where the two constriction gestures have an equal degree of constriction:

both are stops. Several African languages have "labio-velar" stops -- doubly articulated bilabial and velar stops. For example, the name of the language Igbo has three segments: the two vowels and the consonant [ɡ͡͡b].
both are fricatives. A simultaneous [ʃ] and [x] occurs in so vastly many of the world's languages (namely Swedish) that IPA gives it its own symbol: [ɧ].

Double articulations without their own symbol can be transcribed by giving the symbols for each articulation and putting the tie symbol above them.

Secondary articulations

Secondary articulations are approximants that are articulated at the same time as a stop or a fricative (or a lateral approximant), which is the primary articulation. Secondary articulations are often closely related to vowels.

The common secondary articulations are:

labialization, a simultaneous [w], as in [tʷ].
palatalization, a simultaneous [j], as in [tʲ].
velarization, a simultaneous [ɰ], symbolized by a superscript Greek letter gamma (the voiced velar fricative), as in [tˠ].
pharyngealization, a simultaneous pharyngeal approximant, symbolized by a superscript [ʕ] (the voiced pharyngeal approximant), as in [tˤ]. Velarization and pharyngealization can also both be marked by a tilde through the consonant, as we have seen in the symbol for the dark L, [ɫ].

English consonants can have secondary articulations through assimilation to a neighbouring vowel. It is possible for languages to contrast consonants with a secondary articulation and consonants without.

Slavic languages typically contrast palatalized consonants with non-palatalized (sometimes velarized) consonants. So does Gaelic.
Arabic contrasts pharyngealized and non-pharyngealized consonants.
Nisga'a contrasts labialized and non-labialized (often palatalized) velar consonants. So did proto-Indo-European.

The "other symbols" section of the IPA chart lists two symbols [ɕ] and [ʑ] as "alveolo-palatal fricatives". These can be seen as simple postalveolar fricatives with palatalization as a secondary articulation. They could just as easily be transcribed as [ʃ ʲ] and [ʒ ʲ]. These sounds occur in Polish (where they contrast with ordinary [ʃ] and [ʒ]) and in Mandarin Chinese (where [ɕ] contrasts with retroflex [ʂ]).

Handy conversion chart for the Pinyin system of romanizing Chinese:

	alveolar		retroflex		palatalized postalveolar
fricative	s	[s]	sh	[ʂ]	x	[ɕ]
unaspirated affricate	z	[ts]	zh	[ʈʂ]	j	[tɕ]
aspirated affricate	c	[tsʰ]	ch	[ʈʂʰ]	q	[tɕʰ]