In another chapter reference has been made to the fact that in order to solve the internal structure of a crystal it is necessary to know its approximate chemical composition, the dimensions of the unit cell, the diffraction intensities, and the symmetry of the crystal. Well, it seems reasonable to assume that the internal symmetry of a crystal (the space group) could hardly be inferred from observing its external habit. In fact, the crystal symmetry can only be deduced from the diffraction pattern itself, since certain crystal symmetry operations leave some type of trace in the diffraction pattern, systematically canceling certain intensities that follow the rules that are shown below (some of these rules were explained in another section):

---

---

But let's go back...
 

---