You can distinguish P( primitive), I (bcc) and F (fcc) lattices from the spacings of their innermost 7 allowed reflections:

P: allowed are all, i.e. 100, 110, 111 ,200, 210, 211, 220 etc;

for cubic giving ratios of 1 : root2 : root3 : 2 : root5 : root6 : 2root2=2.82...

I: allowed are 110, 200, 211, 220 ,310, 222, 321 etc

for cubic giving ratios of 1 : root2 : root3 : 2 : root5 : root6 : root7=2.64...

F: allowed are 111, 200, 220 etc: for cubic giving ratios of 1:root(4/3),root(8/3),...

You can see that for F you only need to meaure the 3 innermost reflections, but to distinguish P & I you really need all 7, and need to measure the last spacing to better than 2% accuracy (no problem in XRD).

You can distinguish P( primitive), I (bcc) and F (fcc) lattices from the spacings of their innermost 7 allowed reflections:

P: allowed are all, i.e. 100, 110, 111 ,200, 210, 211, 220 etc;

for cubic giving ratios of 1 : root2 : root3 : 2 : root5 : root6 : 2root2=2.82...

I: allowed are 110, 200, 211, 220 ,310, 222, 321 etc

for cubic giving ratios of 1 : root2 : root3 : 2 : root5 : root6 : root7=2.64...

F: allowed are 111, 200, 220 etc: for cubic giving ratios of 1:root(4/3),root(8/3),...

You can see that for F you only need to meaure the 3 innermost reflections, but to distinguish P & I you really need all 7, and need to measure the last spacing to better than 2% accuracy (no problem in XRD).

Even simple, the question is a good one and the answer by Thomas Walther is quite clear. For a completed understanding and practice of this "sport" I like recommend the attached practice file.

Regards

Daniel