Symmetrical voice, also known as Austronesian alignment, the Philippine-type voice system or the Austronesian focus system, is a typologically unusual kind of morphosyntactic alignment in which "one argument can be marked as having a special relationship to the verb". This special relationship manifests itself as a voice affix on the verb that corresponds to the syntactic role of a noun within the clause, that is either marked for a particular grammatical case or is found in a privileged structural position within the clause or both.
Symmetrical voice is best known from the languages of the Philippines, but is also found in Taiwan's Formosan languages, as well as in Borneo, Northern Sulawesi, and Madagascar, and has been reconstructed for the ancestral Proto-Austronesian language.
The term Austronesian focus was widely used in early literature, but more scholars turn to the term voice recently because of the arguments against the term 'focus'. On the other hand, Starosta argued that neither voice nor focus is correct and that it is a lexical derivation.
Schachter (1987) proposed the word 'trigger', which has seen widespread use. As one source summarized, 'focus' and 'topic' do not mean what they mean in discourse (the essential piece of new information, and what is being talked about, respectively), but rather 'focus' is a kind of agreement, and the 'topic' is a noun phrase that agrees with the focus-marked verb. Thus using those terms for Austronesian/Philippine alignment is "misleading" and "it seems better to refer to this argument expression as the trigger, a term that reflects the fact that the semantic role of the argument in question triggers the choice of a verbal affix."
A number of studies focused on the typological perspective of Austronesian voice system.
Some explored the semantic or pragmatic properties of Austronesian voice system.
Others contributed to the valence-changing morphology.
In languages that exhibit symmetrical voice, the voice affix on the main verb within the clause marks agreement with "the semantic role of the [subject]".