1. After transforming the zone axis uvw of a FCC crystal into utvtwt of its twin or u′v′w′ of the HCP phase formed in it, it is possible to recognize immediately from these indices whether the electron diffraction pattern of the latter is distinct from that of the former.2. The qpndition for the twin to have a non-distinct pattern is: u(t2)+v(t2)+w(t2)>u2+v2+w2. 3. The condition for the HCP phase to have a non-distinct pattern is: u′,v′≠3n, u′+v′=3n. 4. As far as the low zone indices (u, v, w≤4) electron diffraction patterns of a FCC crystal are concerned, approximately 80% of the twin or HCP phase are not discernible by electron diffraction analysis.