Every earlier Miller-index row in this book held two indices at zero; three genuinely mixed index triples still divide the same lattice constant exactly. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Chapter six's rows all kept two indices at zero

Every earlier Miller-index row in this book held k and l at zero and only changed h. A real plane family can carry all three indices at once: h=2, k=3, l=6 still gives an exact spacing because 49 is a perfect square.

h2+k2+l2=49=72,d=127 mh^{2}+k^{2}+l^{2}=49=7^{2},\quad d=\tfrac{12}{7}\ \text{m}
Miller spacingA perfect-square Miller family gives an exact plane spacing.spacingplane family

Three genuinely three-dimensional plane families

Each row uses a different exact Pythagorean-style index triple, not just a scaled single axis. The spacing still divides the same lattice constant exactly in every row.

hkld236127 m14843 m34121213 m\begin{array}{c|c|c|c}h&k&l&d\\2&3&6&\tfrac{12}{7}\ \text{m}\\1&4&8&\tfrac{4}{3}\ \text{m}\\3&4&12&\tfrac{12}{13}\ \text{m}\\\end{array}
Miller spacingA perfect-square Miller family gives an exact plane spacing.spacingplane family

A larger index triple still divides the lattice exactly

The third row uses h=3, k=4, l=12. Even with larger indices, 169 is still an exact perfect square, so the spacing stays exact.

d=12 m169=1213 md=\frac{12\ \text{m}}{\sqrt{169}}=\tfrac{12}{13}\ \text{m}
Miller spacingA perfect-square Miller family gives an exact plane spacing.spacingplane family