Since its release in 2007, the iPhone has undergone three major hardware revisions. Each different generation of iPhone has its own telling markers that help you to distinguish it from the others. If you want to determine what type of iPhone you have, you can do so in a few simple steps by examining your device.
Turn the iPhone over so that the backside of the device is facing up. The backside of the iPhone is the most telling indicator of the device's generation.
Examine the shape of the back of the device. If the back is smooth and flat, without any curve or tapering around the edges, it is a fourth-generation iPhone, designated the iPhone 4. If it has some curve or tapering around the edges, it is one of the earlier generations.
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Examine the material the back of the iPhone is made of if the back is curved. If the back is made of aluminum, it is a first generation iPhone. If the back is made of plastic, it is either a second- or third-generation iPhone.
Examine the imprint on the back of the iPhone if the back is made of plastic. If the entire imprint is bright silver, the iPhone is a third-generation iPhone, designated iPhone 3GS. If only the Apple logo on the back of the device is bright silver, the iPhone is a second-generation iPhone, the 3G.