Duality principle or principle of duality may refer to: Duality (projective geometry) Duality (order theory) Duality principle (Boolean algebra) Duality principle for sets Duality principle (optimization theory) Lagrange duality Duality principle in functional analysis, used in large sieve method of analytic number theory Wave–particle duality See also Duality (mathematics) Duality (disambiguation) Dual (disambiguation) List of dualities