These three precession photos of an orthorhombic crystal clearly show the three mutually perpendicular mirror planes:

The first picture shows the two mirror planes perpendicular to b* and to c*, the second shows the mirrors perpendicular to a* and to c*, the third shows the mirrors perpendicular to a* and to b*. The Laue symmetry of an orthorhombic crystal is mmm and all eight classes of relections are symmetry-related:



Only ¹/₈ of the total data need be collected to obtain a complete set of unique data.