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In crystallography there are conventions as to how the indices of planes and directions are written. When referring to a specific plane, "round" brackets are used:
When referring to a set of planes related by symmetry, then "curly" brackets are used:
These might be the (100) type planes in a cubic system, which are (100), (010), (001), (100),(010) and (001) . These planes all "look" the same and are related to each other by the symmetry elements present in a cube, hence their different indices depend only on the way the unit cell axes are defined. That is why it useful to consider the equivalent (010) set of planes.
Directions in the crystal can be labelled in a similar way. These are effectively vectors written in terms of multiples of the lattice vectors a, b, and c. They are written with "square" brackets:
\[[U V W]\]
A number of crystallographic directions can also be symmetrically equivalent, in which case a set of directions are written with "triangular" brackets: