.. include:: /substitutions.rst

.. _convention_coordinates:

Coordinate system
-----------------

The lattice vectors describing the unit cell are named |a|, |b|, and |c|.
|a| and |b| must be in the :math:`(x, y)` plane, and |c| must have a positive
:math:`z` component. The :math:`+z` direction is considered to be pointing
outwards from the surface (i.e., towards vacuum). For (1 × 1) cells, it
is common (and highly recommended) to choose the |a| and |b| vectors such that
:math:`|\mathbf{a}| \leq |\mathbf{b}|`.