Example: "An egg with a double yolk"
Example: "A double (or dual) role for an actor"
Example: "Ancient greek had the dual form but it has merged with the plural form in modern greek"
Of an item that is one of a pair, the other item in the pair.
Of a regular polyhedron with v vertices and f faces, the regular polyhedron having f vertices and v faces.
Example: "The octahedron is the dual of the cube."
(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.
Of a vector in an inner product space, the linear functional corresponding to taking the inner product with that vector. the set of all duals is a vector space called the dual space.