A natural coupling is a coupling between classes which represent concepts which are dependent in the same way.
The following couplings are natural ones:
Library depending on BookStopwatch depending on TimeOrder depending on OrderItemOrderProxy depending on OrderThe following couplings are artificial:
Time depending on Stopwatch (time should be meaningful without a stopwatch)OrderItem depending on Order (an order item way at least be viewed separately from the order whereas an order without any items is not an order anymore)Order depending on DatabaseConnection (an order which is not persisted is normally still an order, the dependency is not necessary from a conceptual point of view)Order depending on OrderProxyDiscuss this wiki article and the term on the corresponding talk page.