Huntington, Samuel P. "Reform and Stability in South Africa." International Security v. 6, n. 4 (1982): 3-25.

“Multi-ethnic or multi-racial systems can be classified as vertical or hierarchical, on the one hand, and horizontal or parallel on the other. In a vertical or hierarchical system, “stratification is synonymous with ethnicity”; in a horizontal or parallel system, different ethnic communities coexist side-by-side, each with its own stratification system independent of the other. Multi-ethnic societies often combine elements of both hierarchy and parallelism and over time, one type of system may change into another.”

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