Which term describes the reference system used to describe the symmetry and shape of crystals?

• A. Crystal Planes
• B. Glide Plane
• C. Crystal Axes
• D. Crystal Shape (form)

Answer: (C)

The reference system used to describe a crystal’s symmetry and shape is the crystal axes. They provide the coordinate framework (the a, b, c directions and the angles between them) needed to specify directions, planes, and how the crystal repeats its pattern. From these axes, you define the unit cell and how features relate to the overall symmetry. Crystal planes describe faces using Miller indices, glide planes are a type of symmetry operation, and crystal shape (form) refers to the external outline rather than the underlying coordinate system. So crystal axes is the term that describes the reference system.