In the cellular concept, frequencies allocated to the service are re-used in a regular pattern of areas, called 'cells', each covered by one base station. In mobile-telephone networks these cells are usually hexagonal. In radio broadcasting, a similar concept has been developed based on rhombic cells. To ensure that the mutual interference between users remains below a harmful level, adjacent cells use different frequencies. A set of C different frequencies {f₁, ..., f_C} are used for each cluster of C adjacent cells. Cluster patterns and the corresponding frequencies are re-used in a regular pattern over the entire service area. The total bandwidth for the system is C times the bandwidth occupied by a single cell.

In the practice of cell planning, cells are not hexagonal as in the theoretical studies. The closest distance between the centres of two cells using the same frequency (in different clusters) is determined by the choice of the cluster size C and the lay-out of the cell cluster. This distance is called the frequency 're-use' distance. It can be shown that the reuse distance r_u, normalized to the size of each hexagon, is r_u = SQRT{3 C}

Thus CLUSTER refers to the set of different frequencies used in a group of cells. The CLUSTER is repeated by linear shift of i steps along one direction and j steps along other direction. For hexagonal cells, i.e., with 'honeycomb' cell lay-outs commonly used in mobile radio, possible cluster sizes are C = i² + ij + j², with integer i and j (C = 1, 3, 4, 7, 9, ...). Integers i and j determine the relative location of co-channel cells.