[space group reference] : [transform] > [transform] > [space group reference] : [transform] which is referred to as a "CLEG expression". (A CLEG setting is the simplest form of a CLEG expression.) This paper details exactly how these three aspects are to be described for CLEG notation and then demonstrates their utility using examples from their implementation in Jmol . Appendix A describes CLEG in formal grammar notation with inclusions of crystallographic definitions from the SymCIF and MagCIF dictionaries as needed.