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The letters S, A, and O have been used heuristically for distinguishing ergative-absolutive languages from nominative-accusative languages. This schema, however, has serious disadvantages for the understanding of individual grammars and even more for broad typological work, because it obscures the incommensurable ways in which participants may be related to events or states. Three of these ways are described here and their incommensurability demonstrated. One is the starting point function, reflected in grammatical subjects; another consists of the semantic roles that are reflected in grammaticized agent-patient marking; the third is immediacy of involvement, reflected in absolutive marking. These relations may be cognitively available to speakers of all languages, and are often grammaticized in different parts of the grammar of a single language. Now that more is known about ways in which languages vary, it is time to sharpen our tools so that we may move on to understanding the forces that shape the grammatical structures we find.
Mithun et al. (Fri,) studied this question.