Gapping removes the finite element (T and its host), possibly along with additional material, in the second and subsequent coordinates of a coordination structure, leaving behind two remnants. (The material that has gone missing—in (1), the finite auxiliary had and the main verb ordered—is represented with ∆.)
Building on a long tradition of earlier work, Johnson (2009:293) identifies three unique properties that distinguish gapping from superficially similar elliptical constructions, such as pseudogapping (e.g., Some had ordered mussels, and others had ∆ swordfish). First, gapping is restricted to coordination structures (2) (Jackendoff 1971:22, Hankamer 1979:18–19). Second, the gap in gapping cannot be embedded (3) (Hankamer 1979:19). Third, the antecedent in gapping cannot be embedded (4) (Hankamer 1979:20).
Crucially, (4) is ungrammatical under an interpretation in which only the antecedent clause—not the gapped...