By considering the ratio of the intensities of weak superlattice reflexions to those of ordinary reflexions, averaged over limited ranges of Bragg angle, an estimate has been obtained for the r.m.s, magnitude of the atomic displacements from the pseudo-structure which give rise to the weak reflexions.

It is shown by enumeration that in four dimensions there are 52 translation groups (Bravais lattices). 21 of these are primitive and 31 are centred.

Standard symbols representing crystal families, twoand three-dimensional Bravais-lattice types and arithmetic classes are recommended for use by the IUCr. The six crystal families are designated by lower-case letters. The family letter in the symbol of each of the 14 lattice types is followed by an upper-case letter to distinguish different lattice types within the family. Arithmetic classes are indicated by modified symbols of the corresponding symmorphic space groups.* Appointed 18 March 1980 under ground rules outlined in Acta Cryst. (1979) 0108-7673/85/030278-03501.50 Crystal familiesSix lower-case letters, which originate from words denoting the shape of commonly used unit cells, are recommended as the standard designation of crystal families (Table l) Adoption of these symbols yields a clear interpretation of the lattice symbols, see below. Direct applications are not envisaged but future use is likely, since the ambiguity of the term 'crystal system' (see above) has so far prevented the general acceptance of a suitable nomenclature for this scale of classification. No such difficulty exists with the term 'crystal families'. Bravais-lattice typesSymbols for designating Bravais-lattice types independently of any description by a particular unit cell with labelled axes are presented in Table 2. The initial lower-case letter characterizes the crystal family (see above) to which the Bravais-lattice type belongs. The second letter designates the type of centring. These symbols are identical with those listed in IntePnational Tables for Crystallography (1983) and used in the 60-year index to Structure Reports ( 1976),* with the exception of mS and oS, which replace mC and pC.The letter C in the monoclinic and orthorhombic Bravais-lattice-type symbols is ambiguous since C is closely associated with one of the common setting and/or unit-cell choices for the lattice, namely A, C or I centring for the monoclinic family and A, B or C centring for the orthorhombic family. The Ad-Hoc Committee has chosen the latter S, derived from 'side-face centred ' (i.e. seitenfliichenzentriert), to replace C. This letter, which is also used to designate similarly centred four-dimensional lattices (Wondratschek, Biilow& Neubiiser, 1971), is clearly appropriate for oS-type lattices. In the case of mS-type lattices, the letter S evokes neither an A-nor a C-centred in preference to an /-centred unit cell: the symbol for the monoclinic centred lattice remains mS regardless of the basis actually used.A similar comment applies to the symbol hR, which designates the rhombohedral lattice regardless of whether that lattice is described (using rhombohedral * The first use of this nomenclature was in an example cited by Laves (1966) as chairman of the IUCr Subcommittee on Structure Type Designation. All 14 symbols were published and extensively used by Pearson (1967). Arithmetic classesThe Ad-Hoc Committee recommends that an arithmetic class be designated by the modified symbol of the symmorphic space group contained in that class. T...

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