are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens. fac, mer The terms fac (from Latin facies) and mer (from meridonal) can specify the arrangement of three identical ligands around the central atom in octahedral complexes. Today, this nomenclature is considered obsolete, but is still permissible. The prefix fac describes the situation when the three identical ligands occupy the three vertices of an octahedron triangular surface. In mer configuration the three ligands span a plane in which the central atom is located. fac and mer are prefixed in small and italic to the complex name. n, iso, neo, cyclo The prefixes n (normal), iso (from Greek ísos = equal), neo (Greek néos = young, new) and cyclo (Greek kyklos = circle) are primarily used to describe the arrangement of atoms, usually of carbon atoms in carbon skeleton. n, iso and neo are no longer used in the systematic nomenclature, but still frequently in trivial names and in laboratory jargon. The prefix n describes a straight-chain carbon skeleton without branches, whereas iso describes a branched skeleton, without specifying any further details. More generally, iso is a compound which is isomeric to the n compound (a compound in which individual atoms or atomic groups are rearranged) neo is a non-specific term for "new", usually synthetically produced substances or isomers of long-known n compounds or natural substances (for example neomenthol derived from menthol or neoabietic acid from abietic acid). According to IUPAC neo is only recommended in neopentane or the neopentyl residue. cyclo is a frequently used prefix for all cyclic and heterocyclic compounds. In many proper names of chemical substances cyclo is not used as a prefix but directly part of the name, for example in cyclohexane or cyclooctatetraene. While n, iso and neo are written in small and italic letters, for cyclo this is only the case in inorganic compounds. In organic compounds, "cyclo" is frequently used as a name component, not separated by a hyphen and also considered in alphabetical sorting. sec-, tert- The prefixes sec and tert are used to indicate the substituent environment in a molecule. Thus, not the exact position of the substituent is described but only the substitution pattern of the adjacent atom (usually a carbon atom). In n-butanol, the OH group is attached to a primary carbon atom, in sec-butanol to a secondary carbon