Differential graded Koszul duality: an introductory survey

This is an overview on derived nonhomogeneous Koszul duality over a field, mostly based on the author’s memoir arXiv:0905.2621. The paper is intended to serve as a pedagogical introduction and a summary of the covariant duality between DG-algebras and curved DG-coalgebras, as well as the triality between DG-modules, CDG-comodules, and CDG-contramodules. Some personal reminiscences are included as a part of historical discussion.