Quartz crystal has a hexagonal prism structure.
Z-axis runs along the length of the crystal and is called optical axis.
The physical properties repeat themselves after every 1200 rotation about Z-axis.
The X-axis or the electric axis is parallel to a line bisecting the angles between adjacent prism faces.
Electric polarization occurs along this direction when the mechanical pressure is applied.
The Y-axis, which is also known as the mechanical axis, is at right angles to the face of prism and also to the X-axis.
The deformation caused due to electric field is seen in this direction.