Numbering of the sides of a cube

In many situations, especially in continuum mechanics, it is useful to have a generic way to refer to the different sides of a cube.

When the lines are parallel to a vector base $({\vec {e_{1}}},{\vec {e_{2}}},{\vec {e_{3}}})$, then :

• the sides perpendicular to ${\vec {e_{i}}}$ ares called "i" and "-i" ;
• point from the center of the cube, ${\vec {e_{i}}}$ points towards the i side, the -i side is at the opposite.