To study and understand the crystal faces, different types of parameters are used. The parameters of crystal faces may be defined as the intercepts made by the crystal face on the crystallographic axes. Parameters are expressed in terms of the unit lengths. Consider the axes shown in the following figure, a,b and c are the unite lengths. A crystal face PQR intercepts the axes at distances of 3a, 3b, and 2c respectively from the origin, so its parameters will be 3a:3b:2c. The form whose face is taken to be making unit parameters is called a ”unit form”.