# Direction indices

### From Online Dictionary of Crystallography

Indices de direction (*Fr*); Indices de dirección (*Sp*); Indici di direzione (*It*); 方向指数 (*Ja*).

Any direction in a Bravais lattice is identified by a set of N indices (where N is the dimensionality of the space), written in square brackets, called the **direction indices**.

In the three-dimensional space, the direction passing through the origin and the lattice nodes *nu*,*nv*,*nw*, where *n* is an integer, has direction indices [*uvw*]. This corresponds to taking the coordinates of the first lattice node on that direction after the origin as direction indices. The indices [-*u*, -*v*, -*w*] identify the same direction as [*uvw*] simply observed from the opposite side.

When a primitive unit cell is used, the direction indices are all integer; they may instead be rational when a centred unit cell is adopted.