A modal logic is called invariant if for all automorphisms of NExt K, () = . An invariant logic is therefore uniquely determined by its surrounding in the lattice. It will be established among other that all extensions of K.alt1, S4.3 and G.3 are invariant logics. Apart from the results that are being obtained, this work contributes to the understanding of the combinatorics of finite frames in general, something wich has not been done except for transitive frames. Certain useful concepts will be established, such as the notion of a d