In the following by 'crystal' we mean any solid having an essentially discrete diffraction diagram, and by 'aperiodic crystal' we mean any crystal in which three-dimensional lattice periodicity can be considered to be absent. As an extension, the latter term will also include those crystals in which three-dimensional periodicity is too weak to describe significant correlations in the atomic configuration, but which can be properly described by crystallographic methods developed for actual aperiodic crystals.

(1) To promote the development of common methods and nomenclature for the crystallographic investigation of aperiodic crystals, including modulated structures, polytypes, incommensurate misfit or composite crystals, and quasicrystals.

(2) To promote and coordinate scientific interchange among groups working in the field of incommensurate structures, quasicrystals, polytypes and periodic crystals in general. In particular, to promote the dissemination of the existing methods for structural analysis, and support the development of new ones.

(3) To promote the exchange of adequate samples and/or experimental data sets of these crystals, so that they can be made available to a larger number of groups.

(4) To cooperate with the Commission on Crystallographic Nomenclature and other interested groups in the development of a standard nomenclature for dealing with higher-dimensional descriptions of aperiodic crystals.

(5) To promote cooperation between crystallographers working on aperiodic crystals and mathematicians working in the field of high-dimensional symmetry groups and/or aperiodic systems, and to stimulate the development of mathematical crystallography.

(6) To promote knowledge among crystallographers of superspace or high-dimensional-space methods as tools which can also be used to advantage in investigation of certain periodic crystal structures.

(7) To cooperate with other Commissions of the IUCr in establishing adequate guidelines and standards for articles to be published in IUCr journals reporting structural investigations of aperiodic crystals and theoretical investigations of aperiodic patterns.

(8) To promote and coordinate scientific meetings dealing with aperiodic crystals; in particular to continue the triennial series of meetings begun by the preceding Commission.

(9) To advise the IUCr on organizing or sponsoring sessions on aperiodic crystals and/or high-dimensional crystallography at Congresses.

During the year the Commission actively supported and helped organize two meetings. In April-May 1991, in Lekeitio, Spain, there was an International Workshop on Methods of Structure Analysis of Modulated Structures and Quasicrystals, which focused on the methods of structure determination of both incommensurate structures and quasicrystals. In August 1991, also under the auspices and support of the Commission, MOSPOQ 91 (International Conference on Polytypes, Modulated Structures and Quasicrystals) was held in Balatonsz6plak, Hungary, in which the whole field of aperiodic crystals was represented with a multidisciplinary viewpont. Both meetings were attended by more than 100 participants. On both occasions a majority of the members of the Commission were present and met to discuss its activities.

Following its terms of reference, the Commission has initiated plans for an international meeting on Aperiodic Crystals in Lausanne, Switzerland, in 1994. This meeting will continue and substitute for, on a triennial basis, the previous MOSPOQ meetings.

At the moment, the elaboration of a set of guidelines about the contents to be included in structural reports of incommensurately modulated structures and incommensurate composite crystals is a priority task. The Commission is also preparing a mailing list of all people interested and working in the field of aperiodic crystals. Personal requests to the Commission for inclusion on this mailing list are welcomed.