Residual zone

A residual zone is an area whose geographical characteristics are conducive for the survival of many language families in a small area (as opposed to a spread zone). Typical residual zones are mountain ranges and islands. The Caucasus, where three unrelated indigenous language families with together about 40 languages exist in an area the size of France, and New Guinea with about 1000 languages in more than 20 families, are famous examples of residual zones.