In linguistics, ellipsis (from , élleipsis 'omission') or an elliptical construction is the omission from a clause of one or more words that are nevertheless understood in the context of the remaining elements. There are numerous distinct types of ellipsis acknowledged in theoretical syntax. Theoretical accounts of ellipsis seek to explain its syntactic and semantic factors, the means by which the elided elements are recovered, and the status of the elided elements. Theoretical accounts of ellipsis can vary greatly depending in part upon whether a constituency-based or a dependency-based theory of syntactic structure is pursued. Background Varieties of ellipsis have long formed a basis of linguistic theory that addresses basic questions of form–meaning correspondence: in particular, how the usual mechanisms of grasping a meaning from a form may be bypassed or supplanted via elliptical structures. In generative linguistics, the term ellipsis has been applied to a range of syntax in which a perceived interpretation is fuller than that which would be expected based solely on the presence of linguistic forms. One trait that many types and instances of ellipsis have in common is that the appearance of ellipsis is optional. The occurrence of VP-ellipsis, for instance, is often optional, e.g. He will help, and she will (help), too. Whether or not the second occurrence of the verb help is elided in this sentence is up to the speaker and to communicative aspects of the situational context in which the sentence is uttered. This optionality is a clear indication of ellipsis. At other times, however, ellipsis seems to be obligatory, for instance with cases of comparative deletion, e.g. *More girls were there today than girls were there yesterday. The second occurrence of girls must be omitted in this sentence (More girls were there today than were there yesterday). The obligatory occurrence of ellipsis complicates the analysis, since one can argue that obligatory cases are not really instances of ellipsis at all, but rather a null pro-form is involved. These aspects of the theory should be kept in mind when considering the various types and instances of ellipsis enumerated below. Types There are numerous widely acknowledged types of ellipsis. They include, as mentioned and briefly illustrated below: Gapping Stripping Verb phrase ellipsis Pseudogapping Answer ellipsis Sluicing Nominal ellipsis Comparative deletion Null complement anaphora Among experts, there is no unanimity that all of the abovementioned syntaxes form a natural class in the sense