a system for intentifying planes and directions in any crystal system by means of sets of four indices (of the type h,k,i,l) in hexagonal system and which involve the use of a fourth axis, coplanar with and at 120o to a1, a2 and a3 (compare: Miller indicies), indeksite süsteem (neljaindeksiline h,k,i,l) kristallograafiatasandite ja -suundade tähistamiseks heksagonaalses süsteemis, kus on lisatud neljas telg, mis on koplanaarne baastasandil kolme 120o-se nurga all orienteeritud telgega a1, a2, a3 (vrd Milleri indeksid)