“The system Back key is used to navigate based on the history of screens the user has recently seen, in reverse chronological order—in effect, the temporal relationships between screens. When the previously viewed screen is also the hierarchical parent of the current screen, pressing the Back key will have the same result as pressing an Up button — this is a common occurrence. However, unlike the Up button, which ensures the user remains within your app, the Back key can return the user to the Home screen, or even to a different application.”
- Android Design Guidelines
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