update / Twinning

[Howard Flack]

> There was an informal meeting of the people formulating items for
> TWINNING at the ACA meeting in Cincinnati.  The names of some items have
> been proposed, but there are still some conceptual problems related to
> the labelling of reflections that need to be worked out.  Victor Young is
> leading this group.

  I would draw the attention of this group to the publication Acta Cryst
(2002). A58, 605-621:

    Report of a Subcommittee on the Nomenclature of n-Dimensional
Crystallography. II. Symbols for arithmetic crystal classes, Bravais
classes and space groups1 by T. Janssen,a*+ J. L. Birman,b F. Dénoyer,c
V. A. Koptsik,d J. L. Verger-Gaugry,e D. Weigel,f A. Yamamoto,g S. C.
Abrahamsh++ and V. Kopskyi 

  Of particular importance to notions of twinning are the definitions of
'holohedry' and 'Bravais group' that are defined in the above
publication. As an example, for a crystal structure in space group Pc
with cell dimensions a = 5.25, b =
6.39, c = 8.12, alpha = beta = gamma = 90 degrees the holohedry is mmm
and the Bravais group is 2/m. (The subcommittee's chairman, Ted Janssen,
confirms this interpretation.)
  Any use of terms such as holohedry, holohedric, holohedral, merohedry,
merohedric and merohedral and versions with pseudo- in front of them
need extreme care if they are to mean anything at all.

H.