New guidelines for the nomenclature of polymorphs and polysomes have been approved by the the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC). Several cases can be distinguished. (i) Polymorphs with different crystal systems are distinguished by the prefixes cubo- (cubic), hexa- (hexagonal), tetra- (tetragonal), trigo- (trigonal), ortho- (orthorhombic), clino- (monoclinic) and anortho- (triclinic). (ii) Polymorphs with different crystal systems but showing a pseudosymmetry should show the prefix ‘pseudo-’. (iii) Polymorphs with the same crystal system but different space groups are distinguished by the prefix ‘para-’. If three or more polymorphs show the same crystal system but different space groups, the space group notation may be added as a suffix, though such a nomenclature should be avoided if possible. (iv) Polymorphs with the same space group are distinguished by the prefix ‘para-’. (v) Minerals with polymorph suffixes but with different chemical compositions cannot be considered as true polymorphs, so we recommend using the prefix ‘meta-’, which indicates a close but significantly different chemical composition. (vi) Polysomatic symbols should be placed as a suffix, which indicates the number and types of modules that alternate in the structure, such as in the högbomite supergroup, or as prefixes as in the sartorite homologous series. These recommendations have to be applied for future new mineral proposals, when the authors decide to use structural prefixes or suffixes, however modifications of historical and well-established names have to pass through the CNMNC for approval. In order to be consistent with the new guidelines, 25 mineral names are now modified: domeykite-β becomes trigodomeykite; fergusonite-(Y)-β becomes clinofergusonite-(Y); fergusonite-(Ce)-β becomes clinofergusonite-(Ce); fergusonite-(Nd)-β becomes clinofergusonite-(Nd); ice-VII becomes cubo-ice; roselite-β becomes anorthoroselite; sulphur-β becomes clinosulphur; mertieite-II becomes mertieite; mertieite-I becomes pseudomertieite; uranophane-α becomes uranophane; uranophane-β becomes parauranophane; gersdorffite-P213 becomes gersdorffite; gersdorffite-Pa3 becomes paragersdorffite; gersdorffite-Pca21 becomes orthogersdorffite; betalomonosovite becomes paralomonosovite; lammerite-β becomes paralammerite; nováčekite-I becomes hydronováčekite; nováčekite-II becomes nováčekite; halloysite-7Å becomes halloysite; halloysite-10Å becomes hydrohalloysite; metauranocircite-I becomes metauranocircite; taimyrite-I becomes taimyrite; uranocircite-II becomes uranocircite; andorite IV becomes quatrandorite; and andorite VI becomes senandorite.