Miller index - Wikipedia
en.wikipedia.org › wiki › Miller_indexthe notation {hkℓ} denotes the set of all planes that are equivalent to (hkℓ) by the symmetry of the lattice. In the context of crystal directions (not planes), the corresponding notations are: [hkℓ], with square instead of round brackets, denotes a direction in the basis of the direct lattice vectors instead of the reciprocal lattice; and