A number of silicon carbide crystals, some new polytypes, have been studied. Phasecontrast microscopic and multiple-beam interferometric techniques have been used to study the growth spiral structure on (0001) and for the measurement of spiral step heights. X-ray methods gave the lattice constants. The various silicon carbide crystals can be divided into the following categories by expressing the relationship between the step height of the growth spirals on (0001) and the measured height of the unit cell along the c axis: (i) crystals in which there is direct correlation between the step height and the unit cell; (ii) those in which the step height is a multiple or submultiple of the unit cell; (iii) a very large unit cell size originating from a complicated growth pattern showing no direct correlation between the step height and the unit cell size; (iv) crystals consisting of a mixture of polytypes and exhibiting an interlaced type of growth spiral; (v) a gradual transition to crystals with unresolved structure and disordered types. Rhombohedral silicon carbide crystals growing from two dislocations, one positive and the other negative, are shown to lead to the creation of a positive and negative rhombohedron (i.e. obverse and reverse positions).