In phonetics, secondary articulation occurs when the articulation of a consonant is equivalent to the combined articulations of two or three simpler consonants, at least one of which is an approximant. The secondary articulation of such co-articulated consonants is the approximant-like articulation. It "colors" the primary articulation rather than obscuring it. Maledo (2011) defines secondary articulation as the superimposition of lesser stricture upon a primary articulation.
There are several kinds of secondary articulation supported by the International Phonetic Alphabet:
It can sometimes be difficult to distinguish primary and secondary articulation. For example, the alveolo-palatal consonants [ɕ ʑ] are sometimes characterized as a distinct primary articulation and sometimes as palatalization of postalveolar fricatives, equivalent to [ʃʲ ʒʲ] or [s̠ʲ z̠ʲ].
The most common method of transcription in the IPA is to turn the letter corresponding to the secondary articulation into a superscript written after the letter for the primary articulation. For example, the w in ⟨kʷ⟩ is written after the k. This can be misleading, as it iconically suggests that the [k] is released into a [w] sound, analogous to ⟨kˡ kⁿ⟩ ([k] with a lateral and nasal release), when actually the two articulations of [kʷ] are generally pronounced more-or-less simultaneously. Secondary articulation often has a strong effect on surrounding vowels, and may have an audible realization that precedes the primary consonant, or both precedes and follows it. For example, /akʷa/ will not generally sound simply like [akwa], but may be closer to [awkwa] or even [awka]. For this reason, the IPA symbols for labialization and palatalization were for a time placed under the primary letter (e.g. ⟨k̫⟩ for [kʷ] and ⟨ƫ⟩ for [tʲ]), and a number of phoneticians still prefer such unambiguous usage, with ⟨kʷ⟩ and ⟨tʲ⟩ used specifically for off-glides, despite the official policy of the IPA. In the official IPA there remains only an alternative symbol for velarization/pharyngealizaton that is superposed over the primary (e.g. ⟨ɫ⟩ for dark L), but that has font support for a limited number of consonants and is inadvisable for others, where it can be illegible. A few phoneticians use superscript letters for offglides and subscript letters for simultaneous articulation (e.g. ⟨tʲ⟩ vs ⟨tⱼ⟩).
There is a longstanding tradition in the IPA that one may turn any IPA letter into a superscript, and in so doing impart its features to the base consonant. For instance, [ʃˢ] would be an articulation of [ʃ] that has qualities of [s]. However, the features are not necessarily imparted as secondary articulation. Superscripts are also used iconically to indicate the onset or release of a consonant, the on-glide or off-glide of a vowel, and fleeting or weak segments. Among other things, these phenomena include pre-nasalization ([ᵐb]), pre-stopping ([ᵖm, ᵗs]), affrication ([tᶴ]), pre-affrication ([ˣk]), trilled, fricative, nasal, and lateral release ([tʳ, tᶿ, dⁿ, dˡ]), rhoticization ([ɑʵ]), and diphthongs ([aᶷ]). So, while ⟨ˠ⟩ indicates velarization of non-velar consonants, it is also used for fricative release of the velar stop (⟨ɡˠ⟩). Mixed consonant-vowels may indicate a transition: [ᵇa] may be the allophone of /a/ with the transition from /b/ that identifies the consonant, while [fʸ] may be the allophone of /f/ before /y/, or the formants of /y/ anticipated in the /f/.
The 2015 edition of the Extensions to the International Phonetic Alphabet formally advocates superscript letters for the first time since 1989, specifically for the release of plosives.