 Hamblin and Prior showed that, in models of linear time, any sequence of temporal operators reduces to a sequence of at most two operators. In total, they identified 15 different such combinations — or tenses, as they call them — that can be expressed in TL over linear flows of time (see Prior 1967, Chapter III.5). Even though these combinations seem to surpass the number of verbal tenses in e.g. English, there are also temporal properties of natural language that cannot be captured in TL: Prior’s temporal operators are, for instance, not very well suited to model distinctions of aspect (e.g. the distinction between “I wrote a letter” and “I was writing a letter”), which are arguably more adequately dealt with in an interval-based framework. For details, see Kuhn and Portner (2006) and the entry on  tense and aspect.