Duality principle
The duality principle in mathematical logic is a theorem on the acceptability of mutual substitution (in a certain sense) of logical operations in the formulas of formal logical and logicalobjective languages. Let be a formula in the language of propositional or predicate logic not containing the implication symbol ; a formula is said to be dual to a formula if it may be obtained from by replacing in each occurrence (cf. Imbedded word) of the symbols , , , by their dual operations, i.e. by the symbols , , , and , respectively. The duality principle states that if is true, then is true as well. In particular, if two formulas and are equivalent, their dual formulas and are equivalent too. The duality principle is valid for classical systems, and the equivalence and the truth of the formulas involved in its formulation may be understood both in terms of interpretations and in the sense of being deducible in the corresponding classical calculus. The duality principle is no longer valid if the formulas are understood in their constructive sense. For instance, in the language of propositional logic the implication is constructively true, and is even deducible in a Heyting formal system, but the converse implication of the dual formula is constructively untrue (it is not Kleenerealizable).
The following theorem is closely connected with the duality principle: If is a formula dual to a propositional or predicate formula constructed without making use of implications from the elementary propositions , then the formula is equivalent to the formula in the classical propositional or predicate calculus, respectively.
References
[1]  P.S. Novikov, "Elements of mathematical logic" , AddisonWesley (1964) (Translated from Russian) 
[2]  S.C. Kleene, "Introduction to metamathematics" , NorthHolland (1951) 
F.A. Kabakov
The duality principle in geometry is a principle formulated in certain fields of geometry, according to which the replacement in any true proposition of all concepts occurring in it by concepts dual to them results in another true proposition, dual to the first.
The validity of the duality principle in projective geometry follows from the fact that to each axiom of projective geometry corresponds a dual proposition which is either an axiom or a theorem.
The following concepts of projective geometry in the plane are dual:' <tbody> </tbody>

If the fact that a point forms part of a secondorder curve is considered to be an incidence relation between the point and the curve, while the fact that a straight line is tangent to a secondorder curve is considered to be an incidence relation between the straight line and the curve, then secondorder curve is a concept dual to that of curve of the second class. The Brianchon theorem and the Pascal theorem are an example of a pair of dual propositions. The concepts of a point and a plane in threedimensional space are dual in projective geometry; the concept of a straight line is dual to itself.
The duality principle is also valid in elliptic geometry, in which the concept of a segment and an angle are dual in addition to those in projective geometry. For instance, the following two statements are dual in elliptic geometry:' <tbody> </tbody>

References
[1]  N.V. Efimov, "Höhere Geometrie" , Deutsch. Verlag Wissenschaft. (1960) (Translated from Russian) 
A.S. Parkhomenko
Comments
Both statements in the main article above concerning triangles in elliptic geometry are false as they stand. They become true if one adds that the triangles must be of the same topological type, cf. [a3].
References
[a1]  E. Artin, "Geometric algebra" , Interscience (1957) 
[a2]  D. Pedoe, "A course of geometry" , Cambridge Univ. Press (1970) 
[a3]  M. Berger, "Geometry" , 1–2 , Springer (1987) (Translated from French) 
The duality principle in projective geometry means that to each theorem about subspaces of a projective space , their intersections and sums there corresponds a theorem concerning the subspaces their sums and intersections. The duality principle is determined by the dual nature of the axioms of projective geometry and the theorems which follow from them. For a projective space over a skewfield the duality principle is valid if and only if permits an antiautomorphism. In the general case there is duality between the projective spaces and whose skewfields and are antiisomorphic; examples are the projective spaces and over (cf. Projective algebra; Correlation), and the correspondence between them, i.e. the correspondence between and , is determined by the choice of a pair of coordinate systems in and . The duality principle can also be based on a dual mapping of the linear spaces over a skewfield, which is used in the interpretation of projective spaces.
M.I. Voitsekhovskii
Comments
References
[a1]  O. Veblen, J.W. Young, "Projective geometry" , 1 , Ginn (1938) 
The duality principle in partially ordered sets means that if some theorem on partially ordered sets, couched in general logical terms and in terms of the order, is true, then its dual theorem is also true. In order to obtain a theorem dual to a given theorem, all propositions and concepts concerned with the order are replaced by their dual ones (i.e. all order signs are replaced by and vice versa), while the general logical terms remain unchanged. The truth of a given proposition concerning a given partially ordered set (or a given class of ordered sets) does not necessarily entail the truth of the dual proposition for this set (class). Thus, a partially ordered set may have a smallest, but not a largest element; it may satisfy the minimality, but not the maximality condition. The truth of the duality principle stems from the fact that the relation inverse to a partial order is itself a partial order; the duality principle itself is sometimes understood to mean this very proposition.
T.S. Fofanova
Comments
A classical example is given by the family of subsets of a set, partially ordered by inclusion.
There is yet another duality principle, generalizing that for partially ordered sets:
5) The duality principle in category theory means that if some general assertion about categories and functors is true, then so is the dual assertion obtained by reversing the direction of the arrows within each category involved (though not the direction of the functors). For example, the theorem that a left adjoint functor is faithful if and only if the unit of the adjunction is monic, is dual to the theorem that a right adjoint is faithful if and only if the counit is epic. (See also Category.)
Duality principle. F.A. Kabakov, A.S. Parkhomenko, M.I. Voitsekhovskii, T.S. Fofanova (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Duality_principle&oldid=17147