The magnetic properties of halides have been compiled in subvolume 27J. The third part, 27J3, contains data on structural and magnetic properties of halide perovskite-type layer structures of the types A2MX4 (monolayer compounds), A3M2X7 (double-layer) and A4M3X10 (triple-layer) where X=F-, Cl-, etc.; M=Mg2+, Cr2+, Mn2+, Zn2+, Cd2+, etc.; and A=K+, Rb+, Cs+, etc. In addition, for X=Cl- and Br- data are presented on compounds where the alkali ions are replaced by n-alkylammonium chains, and also for diammonium compounds where two alkali ions are replaced by divalent alkylene-diammonium ions. A brief sketch of the theory of phase transitions and of the relevant models (2D Ising model, 2D XY-model, 2D Ising spin glass) is also given as well as a comprehensive selection of data on critical properties near the phase transitions.