In any monoclinic crystal, the zone–axis [uvw] is perpendicular to the plane (hkl) if u/(hc2–lac cos β) = v/(kc2a2sin2β) = w/(la2–hac cos β). In particular, the normal to the face (100) is quasi–parallel to the zoneaxis [r01̄] if c/(a cosβ) approximates to a rational number r.

Epidote has a:b:c = 1•5807:1:1•8057,β 115° 24', and is often twinned on a(100). G. Friedel describes and explains these twins in terms of a pseudo–normal zone–axis [301], with an obliquity of 3° and a twinindex 3. But the choice of a pseudo–normal zone–axis is necessarily arbitrary, since it is always possible to find a rational zone–axis making an angle with any given face–normal less than any assigned angle δ: though the indices of such an axis may involve large numbers.