Can I measure dihedral angles between planes?

Yes — planes can be used in distance measurements with other planes, atoms and centroids. Angles between planes can be measured by selecting Measure Angles from the Picking Mode pulldown and clicking on each plane in turn. Note that you can tell when a plane is selected as the corners of the plane will be highlighted with yellow spheres. Also, centroids can be used in distance, angle and torsion angle measurements - this is useful when measuring the distance between two planes which are almost but not quite parallel, which Mercury will always give as zero as the planes will intersect at some point.