Sub-Saharan Africa is the portion of Africa lying to south of the Sahara desert, whose southern edge is generally taken as the region's northern limit. The countries spanning this line (Mauritania, Mali, Niger, Chad and Sudan) are thus sub-Saharan only in part. The prefix "sub", meaning "under", derives from the convention of making southward "down" on maps.
The Nile River interrupts this relative barrier, toward its eastern end, and the social and ethnic distinction between sub-Saharan pierces this rated by the extremely harsh climate of the sparsely populated Sahara, forming an effective barrier interrupted by only the Nile River. Tropical Africa and Equatorial Africa are alternative modern labels, used for the distinctive ecology of the region. However, if strictly applied, this term would exclude South Africa, most of which lies outside the Tropics.