Identification of rotation and reflection twin by diffraction and contrast experiments in the electron microscope

In crystals belonging to certain point-groups two types of twins can occur: rotation and reflection twins. It is not a straightforward matter to distinguish these from electron diffraction and microscopy observations. The present paper deals with the characterisation of these two types of twins in direct space as well as in reciprocal space. The typical electron diffraction aspects are compared, the image characteristics are discussed and practical methods for the distinction between both types of twins are critically analysed with the help of some examples.