Twinned crystals may be described as follows:
Simple twins – composed of only two parts.
Multiple twins – composed of more than two orientations.
Contact twins – this occur if a definite composition plane is present.
Penetration twins – occur if two or more parts of a crystal appear to interpenetrate each other with the surface between the parts being indefinable and irregular.
Polysynthetic twinning – occurs when three or more individuals are repeated alternately on the same twinned plane. If the individuals of polysynthetic twins are thin plates, the twinning is called lamellar e.g. plagioclase feldspars.