The crystal chemistry of the barium fluoride chloride system is studied both experimentally and theoretically. Different synthetic approaches yield nanocrystalline materials as well as large single crystals. The crystalline phases identified so far are BaFCl, Ba₁₂F₁₉Cl₅ and Ba₇F₁₂Cl₂ (in two modifications) and compared with analogous compounds. It is demonstrated that the compound Ba₂F₃Cl reported by Fessenden and Lewin 50 years ago corresponds to Ba₇F₁₂Cl₂. The phase diagram of the BaCl₂ – BaF₂ system is reinvestigated for fluoride mole fractions between 0.5 and 1. The peritectic formation of Ba₁₂F₁₉Cl₅ is observed. Periodic DFT calculations are performed for all structures in this system, including a hypothetical structure for Ba₂F₃Cl, based on the experimental structure of Ba₂H₃Cl. The energy of formation of the different barium fluoride chloride compounds from BaCl₂ and BaF₂ (normalized for one barium atom per formula unit), as calculated by DFT at 0K, is within only about ± 15 kJ/mol. Comparison with recent experimental results on calcium and strontium hydride chloride (bromide) compounds, suggest the possibility of a mutual exclusion between the M₂X₃Y and M₇X₁₂Y₂ (M = Ca, Sr, Ba, Pb, X = H, F, Y = Cl,Br) structures. The single crystal structure of PbFBr is also reported.