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For tetragonal and hexagonal structures an extra variable - the axial ratio,
*c*/*a* - is involved and therefore the problem is more complicated. For
tetragonal structures, eq. (5) is replaced by

(6) |

Now no precise rules can be given, but the values of sin^{2} may give
some hints. For example, if *l* = 0, the values of sin^{2} are in the
ratios 1, 2, 4, 5 corresponding to indices 100, 110, 200, 210 If we
find a set of values in these ratios we can assume that the indices are as
shown, and then we have to find *l* from the lines that are not in this
sequence. The ratio 1:2 particularly should be looked for.

For hexagonal structures, or trigonal structures referred to hexagonal axes, eq. (5) is modified to

(7) |

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