<Li> Tetragonal: <Dl> <Dd> 1 d 2 = h 2 + k 2 a 2 + l 2 c 2 (\ displaystyle (\ frac (1) (d ^ (2))) = (\ frac (h ^ (2) + k ^ (2)) (a ^ (2))) + (\ frac (l ^ (2)) (c ^ (2)))) </Dd> </Dl> </Li> <Dl> <Dd> 1 d 2 = h 2 + k 2 a 2 + l 2 c 2 (\ displaystyle (\ frac (1) (d ^ (2))) = (\ frac (h ^ (2) + k ^ (2)) (a ^ (2))) + (\ frac (l ^ (2)) (c ^ (2)))) </Dd> </Dl> <Dd> 1 d 2 = h 2 + k 2 a 2 + l 2 c 2 (\ displaystyle (\ frac (1) (d ^ (2))) = (\ frac (h ^ (2) + k ^ (2)) (a ^ (2))) + (\ frac (l ^ (2)) (c ^ (2)))) </Dd> <Li> Hexagonal: <Dl> <Dd> 1 d 2 = 4 3 (h 2 + h k + k 2 a 2) + l 2 c 2 (\ displaystyle (\ frac (1) (d ^ (2))) = (\ frac (4) (3)) \ left ((\ frac (h ^ (2) + hk + k ^ (2)) (a ^ (2))) \ right) + (\ frac (l ^ (2)) (c ^ (2)))) </Dd> </Dl> </Li>

How many types of crystal structures are there