A schematic crystal family can make plane spacing an exact arithmetic claim. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A simple family has exact spacing
For this schematic lattice, the lattice constant is 12 meters. The family with h equal to 1 has spacing 12 meters.
d=ha=112m=12m
Doubling the index halves the spacing
With h equal to 2, the same schematic lattice gives spacing 6 meters. Metre-scale lattices are training values, not atomic spacings.