A twin operator may be either reflection on a rational plane or rotation on a rational vector. Normal twins according to the system commonly used for feldspars should be entered as reflection twins, giving the indices of the composition or twin plane. For parallel twins enter the indices of the twin axis as the twin operator.
Indices will be normalized to length one.
For rotation twins, the rotation angle is normally an integral fraction of 360 degrees. A rotation angle is not needed for reflections twins - the angle will be set automatically to 180.
Note that composition planes must always be entered separately, even if the twin is by reflection.