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Apple’s iPod Nano is what I consider to be their main iPod. It’s smaller and cheaper than the Classic, although the storage capacity is less. I own two different model Nanos and love the interface on them. Here’s how to tell them apart:
First Generation (1G) – The original iPod Nano had square edges whereas all the others had smooth rounded edges. This is the best way to tell the first generation models apart from the others.
Second Generation (2G) – Apple got so many complaints about scratched Nanos that they released the second generation model with an aluminum case. You can tell them apart because the screen is smaller than the other models with the metal casing.
Third Generation (3G) – This is the model Nano I own and I love it. This one looks completely different than the other Nanos because it is more square in shape, like a miniature iPod Classic. (See photo.)
Fourth (4G) and Fifth Generation (5G) – The fourth generation models were the first to feature the larger, elongated screens which made them more suitable for watching widescreen movies. The main difference between the 4G and 5G iPod Nanos is that the fifth generation models have a digital camera built in. They also have a slightly larger screen.

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Sixth Generation (6G) - These current models are drastically different from previous versions because they are now entirely touchscreen and much smaller. See image to the right and compare to the one above.
iPod Minis – Don’t confuse the iPod Mini with a Nano since they look like a smaller version of the third generation Nano, but with a monochrome screen. Apple discontinued the iPod Minis in 2005.