deletion Comparative deletion occurs in clauses introduced by than in English. The expression that is elided corresponds to a comparative morph such as more or -er in the antecedent clause, e.g.: More people arrived than we expected people would arrive. She ordered more beer than we could drink beer. Doris looks more satisfied than Doreen looks satisfied. William has friends in more countries than you have friends in countries. Comparative deletion is different from many of the other optional ellipsis mechanisms insofar as it is obligatory. The non-elliptical versions of these sentences are unacceptable. Null complement anaphora Null complement anaphora elides a complete complement, whereby the elided complement is a finite clause, infinitive phrase, or prepositional phrase. The verbal predicates that can license null complement anaphora form a limited set (e.g. know, approve, refuse, decide). The elided complement cannot be a noun phrase. Q: Do you know what happened? A: No, I don't know what happened. Q: Do you approve of the plan? A: No, I don't approve of the plan. They told Bill to help, but he refused to help. They offered two ways to spend the day, but I couldn't decide between them. Of the various ellipsis mechanisms, null complement anaphora is the least studied. In this regard, its status as ellipsis is a point of debate, since its behavior is not consistent with the behavior of many of the other ellipsis mechanisms. Less-studied cases of ellipsis Further instances of ellipsis that do not (in a clear way) qualify as any of the ellipsis types listed above: A: The cat likes Bill. B: Why does the cat [particularly] like Bill? What will happen if I miss the deadline? More work on ellipsis may need to be done before all ellipsis mechanisms are fully explained. Theoretical approaches Ellipsis is widely studied in the theoretical literature, central issues including the mental representation of elided material, the conditions which license ellipsis, and the means by which the elided material is recovered. One challenge to theoretical accounts of ellipsis comes from cases where the elided material does not appear to be a constituents. Since syntactic operations can only target constituents in standard phrase-structural approaches, accounts within these frameworks must posit additional movement operations to explain such cases. These movement rules raise non-elided material out of a constituent, allowing ellipsis to apply only to the material that is left, thus creating the illusion