(grammar) Pertaining to grammaticalnumber (as in singular and plural), referring to two of something, such as a pair of shoes, in the context of the singular, plural and in some languages, trial grammatical number. Modern Arabic displays a dual number, as did Homeric Greek.
(linear algebra) Being the space of all linear functionals of (some other space).
(category theory) Being the dual of (some other category); containing the same objects but with domain and codomain reversed for all functions.
Noun
Of an item that is one of a pair, the other item in the pair.
(geometry) Of a regularpolyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.
(grammar)dual number The grammatical number of a noun marking two of something (as in singular, dual, plural), sometimes referring to two of anything (a couple of, exactly two of), or a chirality-marked pair (as in left and right, as with gloves or shoes) or in some languages as a discourse marker, "between you and me". A few languages display trial number.
