Nullnorms are generalizations of triangular norms and triangular conorms with a zero element in the interior of the unit interval. In this paper, we work on nullnorms which are defined on an arbitrary bounded lattice. We introduce a general median-based method for constructing nullnorms by means of triangular norms and triangular conorms. Furthermore, we highlight a significant difference between the (existence and representation of) (idempotent) nullnorms on chains, distributive bounded lattices and general bounded lattices. (C) 2015 Elsevier B.V. All rights reserved.