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=712m
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.
h213k344l6812d712m34m1312m
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.