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[en] A number of LnOCuCh-related compounds with composition Ln2O2MSe2 (Ln = La & Ce, M = Fe, Zn, Mn & Cd) have been reported in the literature, built from alternating layers of fluorite-like [Ln2O2]2+ sheets and antifluorite-like [MSe2]2– sheets. They all contain divalent transition metal ions leading to half occupancy of tetrahedral sites in the selenide layers. The ordering of the transition metals is different across the known structures: [MSe2]2– layers can either contain MSe4 tetrahedra that are exclusively edge-sharing (E, stripe-like), exclusively corner-sharing (C, checkerboard-like), or sections of both. This work reveals the origins of this ordering by investigating a range of solid solutions. Substitution of M leads to changes almost entirely in the c parameter, perpendicular to the layers, whereas substitution of Ln leads to an approximately isotropic change in all lattice parameters. This is attributed to a relatively rigid Ln-O layer and a flexible M-Se layer, which adapts to the size demands of the Ln-O layer. Transition metal ordering is determined by the relative sizes of [Ln2O2]2+ and [MSe2]2– layers, and can be tuned by doping in either layer. A progressive evolving range of commensurate and incommensurate compounds will be reported showing that the materials can be considered as infinitively adaptive structures.