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(b) The classical ABC notation

As pointed out earlier, the actual arrangement of layers in all close-packed structures can be described in terms of the ABC notation for close-packing of spheres. Thus the SiC type 6H has six Si and six C layers in its hexagonal unit cell stacked as A$\alpha$ B$\beta$ C$\gamma$ A$\alpha$ C$\gamma$ B$\beta$ where the Roman letters denote positions of layers of Si atoms and the Greek letters those of C atoms. Since the positions of C atoms are fixed relative to the positions of Si atoms it is customary to omit the Greek letters and write the structure as ABCACB. In the case of CdI₂ structures, where the Cd atoms lie in the octahedral voids between alternate close-packed iodine layers, one often retains the Greek letters to denote the positions of Cd layers. Thus CdI₂ type 4H has a structure A$\gamma$B C$\alpha$B. While this notation gives a complete description of the structure it does not reveal the symmetry or lattice type directly and becomes cumbersome for structures with large repeat periods.

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