This monograph is intended to give the reader an appreciation of the
wealth of phases, elements and inorganic compounds, which crystallize in
layer-type or two- dimensional structures. Originally this work was
planned as a short review article but the large number of phases made it
grow out to the size of a book. As is evident from the arrangement of
the chapters our point of view was gradually transmuting from geometric
to chemical. Moreover, the decision about the compounds that should be
discussed was taken only during the course of the work, as is partly
evident from the sequence of the references. For chemical or geometrical
reason we have included also certain layered chain and molecular
structures as well as some layered structures whose layers are linked by
hydrogen bonds, thus are in fact three-dimensional. Instead of writing
only a review with pseudo-scientific interpretations that later turn out
to be wrong anyway we thought it more profitable to include the
crystallographic data which are scattered in various original articles
and hand books but never in one single volume. We have transcribed many
of the data in order to make them correspond with the standard settings
of the International Tables for X-Ray Crystallography. The figures are
consistent with the data given in the tables. We apologize for errors
and hope that their number is at a reasonably low level in spite of the
time pressure.