  Characterizations of skeletally Dugundji spaces and Dugundji spaces are given
in terms of club collections, consisting of countable families of co-zero sets.
For example, a Tychonoff space $X$ is skeletally Dugundji if and only if there
exists an additive $c$-club on $X$. Dugundji spaces are characterized by the
existence of additive $d$-clubs.
