The proof is simple Imagine a hexagonal axis system as shown and a face cutting each axis with intercepts a, b, and c. Now construct CE parallel to x1 and CD parallel to x2. Obviously triangles AOB, ADC and CEB are similar. Therefore a/b = {a-c}/c = a/c -1. Dividing by a and rearranging, we have 1/a + 1/b = 1/c Define h = 1/a, k = 1/b and i = -1/c Then h + i + k =0. ================================ Note: In the case of an hexagonal lattice, one uses four axes, [FONT=Times New Roman,Times New Roman]a[/FONT][FONT=Times New Roman,Times New Roman]1[/FONT], [FONT=Times New Roman,Times New Roman]a[/FONT][FONT=Times New Roman,Times New Roman]2[/FONT], [FONT=Times New Roman,Times New Roman]a[/FONT][FONT=Times New Roman,Times New Roman]3[/FONT], [FONT=Times New Roman,Times New Roman]c [/FONT]and four indices, ([FONT=Times New Roman,Times New Roman]hkil[/FONT]), called Bravais-Miller indices, where [FONT=Times New Roman,Times New Roman]h[/FONT], [FONT=Times New Roman,Times New Roman]k[/FONT], [FONT=Times New Roman,Times New Roman]i[/FONT], [FONT=Times New Roman,Times New Roman]l [/FONT]are again inversely proportional to the intercepts of a plane of the family with the four axes. The indices [FONT=Times New Roman,Times New Roman]h[/FONT], [FONT=Times New Roman,Times New Roman]k[/FONT], [FONT=Times New Roman,Times New Roman]i [/FONT]are cyclically permutable and are related by