All iPhone applications can run on the iPad, and not "full-screen" (see the figure below). Applications can be designed for iPhone and iPad separately or a universal application that contains resources for iPhone and iPad (interfaces, device management, etc.).

The user will notice a gray container around the application designed for the iPhone. Universal applications automatically select the correct interface for the device.
In the iTunes Store, applications with a blue plus next to the price are universal applications:

Evan mulawski
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