Every language has intransitive clauses, with a predicate and a single core argument (that we call S) and transitive clauses, with a predicate and two core arguments (A and O). There should always be the means to distinguish A and O. Some languages do this by constituent order (e.g. English), some use cases, particles or adpositions, and some employ pronominal cross-referencing on the verb (many languages employ a combination of these strategies). The marking of core syntactic relations – A, S and O – is generally referred to as ‘morphological ergativity’ or ‘morphological accusativity’ since this is generally shown by case inflections or verbal cross-referencing affixes. A more exact label would be ‘intra-clausal ergativity/accusativity’, since particles and adpositions make use of a syntactic – not a morphological – mechanism, and constituent order is without doubt a matter of syntax.

There must be some means of distinguishing A and O for a transitive clause. The marking of S in an intransitive clause can be the same as A, or the same as O, or different from both. There are thus three basic possibilities:

1. S = O (absolutive), A different (ergative) – an ergative system

2. S = A (nominative), O different (accusative) – an accusative system

3. A, S and O all different – this is a ‘three-way’ or ‘tripartite’ system.

The accusative pattern is, of course, commonest among the languages of the world. The ergative pattern, with which this book is concerned, is by no means uncommon.