The apps you already use on your iPhone will also work on an iPad. However, apps designed for the iPad have a different appearance and require the iPad's larger display. To meet the requirements of both devices, software developers have created "universal" apps that run on an iPhone and also make full use of the iPad's screen.
The design of an iPhone app fits in the 960-by-640-pixel screen used in the iPhone 4 and 4S models and the 1136-by-640-pixel display found in the iPhone 5. Apps employ scroll and pinch techniques to make the small screen more useful. Once you have purchased an iPhone app, you can copy it to an iPad without needing to buy it again. The iPad's zoom feature can expand the app's screen by a factor of two in each dimension, filling up more of the device's larger display area. A button labeled "2X" appears to the side of the app; when you tap it, it makes the app bigger. The button changes to "1X." Tapping it shrinks the size back to iPhone size. In all other respects, iPhone apps work the same way on the iPad.
IPad apps fill out its larger screen size -- 9.7 diagonal inches with a resolution of 2048 by 1536 pixels. They show full-sized documents and Web pages and rely less on scrolling and pinching compared to the iPhone. They have more control buttons and conveniences, such as Safari's tabbed pages. Software vendors offer iPhone versions of their iPad apps, although out of necessity these have compromises to accommodate the smaller screen.
Recognizing that they have customers with both iPhone and iPad devices, app developers sell universal apps. For the price of one download, a universal app accommodates both iPhone and iPad. Used on the iPhone, the app configures itself to use the smaller screen. On its App Store listing, you will see a small "+" symbol near the app's price and rating, indicating a universal app.
The Phone App
One application runs solely on the iPhone: the Phone app. The iPad lacks the cell phone hardware needed to make the Phone app work, so Apple does not include it with the iPad. The Phone app is a factory-installed part of iOS for the iPhone; it is not available through the App Store.
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