Planes with different Miller indices in cubic crystals

Examples of directions

Miller indices form a notation system incrystallography for planes in crystal (Bravais) lattices.

In particular, a family of lattice planes is determined by three integers h, k, and l, theMiller indices. They are written (hkl), and each index denotes a plane orthogonal to a direction(h, k, l) in the basis of the reciprocal latticevectors. By convention, negative integers are written with a bar, as in 3 for −3. The integers are usually written in lowest terms, i.e. theirgreatest common divisor should be 1. Miller index 100 represents a plane orthogonal to direction h; index 010 represents a plane orthogonal to direction k, and index 001 represents a plane orthogonal to l.

There are also several related notations:^[1]

• the 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
• similarly, the notation  denotes the set of all directions that are equivalent to [hkl] by symmetry.

Miller indices were introduced in 1839 by the British mineralogist William Hallowes Miller. The method was also historically known as the Millerian system, and the indices as Millerian,^[2] although this is now rare.

The Miller indices are defined with respect to any choice of unit cell and not only with respect to primative basis vectors, as is sometimes stated.