There are four main sections. The
**Introduction** continues, after the present
subsection, with a discussion
of the rationale for and genesis of the Subcommittee that wrote this
report (Subsection 1.2), notation for position and
displacement vectors (1.3), a consideration of the
structure factor and atomic displacements (1.4), and
comments about the terminology to be used (1.5).

The second main section, **Displacement Parameters
Based on the Gaussian Approximation**, concerns the kinds of parameters
describing atomic displacements that have most often been used in crystal
structure analysis and hence are most commonly found in the literature on
the subject. Its subsections are: 2.1, *Anisotropic
displacement parameters*, which includes a discussion of the common
symbols used; 2.2, *Equivalent isotropic
displacement parameters*; and 2.3, *Graphical
representations of the Gaussian mean-square displacement matrix*.

The third main section, **Beyond the Gaussian
Approximation**, considers the expressions used when the Gaussian
approximation is not adequate. Our **Recommendations
** for nomenclature are in the final main section.