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BELL SYSTEM TECHNICAL JOURNAL 



indices (lOlO). The first two digits of the indices of any vertical prism 

 plane are the same as those of the adjacent cap faces: the final digit is always 

 zero because they are parallel to the Z (or c) axis. Since the intercepts on 

 fli and Qi uniquely determine the intercept on a^, , the third digit of the sym- 

 bol may be omitted. The omission is sometimes indicated by a dot, as 

 (10-1) (general form hk-C). 



0001 



+ a. 



Fig. 3.5 — Miller-Bravais indices of certain crystallographic planes in quartz 



The use of indices instead of the actual intercepts simplifies the mathe- 

 matics of crystallography in many ways. For example, the sum of the 

 indices of two planes forms the indices of an intermediate plane that is 

 parallel to the line of intersection of the first two. Thus the sum of (00-1), 

 the atomic plane which is normal to c, and (10-0), prism face, is (10-1), 

 the indices of the cap face directly above the (10-0) prism face. 



