A new graphic method has been developed for indexing Debye-Scherrer photographs, Utilizing the sin2θ values of three low angle reflexions and the density of the crystal investigated, a series of conditional lines and equiatomic curves are drawn in the A-C space, where A=λ2/4α and C=λ2/4C2 in the tetragonal system, and A=λ2/3a2 and C=λ2/4c2 in the hexagonal system. The conditional lines are a manifold of straight lines representing the general equation miA + niC - sin2θi=0, the intercepts of which on the coordinate axes A and C being xi= sin2θi/mi and yi = sin2θi/ni, where mi= hi2+ ki2, ni = 1i2 in the tetragonal system, and mi = hi2+ hiki + ki2, ni = 1i2 in the hexagonal system. Three manifolds of conditional lines representing respectively three different sin2θi's are drawn in three different colours.The equiatomic curves are represented by the general equation A2C=Kz, where Kz= (1/64) (ρλ3/Mm0)2(1/Z2) for the tetragonal system, and Kz= (1/48)(ρλ3/Mm0)2 (1/Z2) for the hexagonal system, containing Z as a parameter. In each equation, ρ is the density, M is the atomic weight or the formula weight of the crystal, and m0 is the mass of the atom of unit atomic weight. These curves are drawn in black, indicating in each case an integral number of atoms or configuration units contained in the unit cell.The interception of three conditional lines drawn in three different colours, which passes through one of the equiatomic curves, thus determines uniquely the dimension of the unit cell and the number of atoms or configuration units contained therein.This method may be applied to the tetragonal as well as to the hexagonal system. The cubic system may be considered as a special case of the tetragonal system.