Neven Mrgan's tumbl: Labeling the Back button
Most “deep” apps require some amount of navigation, moving the user deeper into child views and then back out to the parent view. That navigational backtracking is typically done with a “Back” button, positioned in the top-left corner, and denoted by a pointed left side. You’ve all seen it:
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Comprehensive post on how to improve the “back” buttons that exist in almost every iOS application. Worth a read.