Abstract: Examples of definitions and statements which exhibit a kind of ‘duality’, such as initial and terminal object and epimorphisms and monomorphisms have been shown. This chapter now considers this duality more systematically. Despite its rather trivial first impression, it is indeed a deep and powerful aspect of the categorical approach. Topics discussed include the duality principle, coproducts, equalizers, and coequalizers. Exercises are provided at the end of the chapter.

Keywords: duality principle, coproducts, equalizers, coequalizers,