Abstract. -The three known varieties of FeS are described and some peculiar features of iron sulfide chnistry are discussed. One introduces the new tool of "similarity operators" for the study of the displacive transitions of FeS (Ta transition) and of MnAs (MnP NiAs type). In pnictides the transitions magnetic -'-non magnetic (MnP type) are first order in conformity with observed changes of volumes and of transition temperatures versus pressure and/or concentration.