It is useful for languages to have a means of distinguishing between subjects and objects, and between arguments A, S, and O. This is helpful so that sentences like "Tom hit Fred" cannot be interpreted as "Fred hit Tom." Tripartite alignment systems accomplish this differentiation by coding S, A and O all differently. However, this is not structurally economical, and tripartite systems are comparatively rare, but to have all arguments marked the same makes the arguments too ambiguous. Alongside the principle of distinguishability seems to operate a principle of economy. It is more efficient to have as few cases as possible without compromising intelligibility. In this way the dual pressures of efficiency and economy have produced a system which patterns two kinds of arguments together a third separately. Both accusative and ergative systems use this kind of grouping to make meaning clearer.