The zone axis (ZA) is the direction lying in all the planes shown in the slice of reciprocal space represented in the diffraction pattern, i.e. the vector perpendicular to all the g vectors in the pattern.

Take any two g vectors, g₁ (h₁k₁l₁) and g₂ (h₂k₂l₂) from the diffraction pattern. The zone axis is given by the vector product::

The zone axis is parallel (or nearly parallel) to the electron beam direction.