Subscriber Identity Module, or SIM, cards are used to act as a unique identifier for a mobile device on a wireless cellular network. They come in multiple size and shapes, but SIM, Mini-SIM and Nano SIM are really the same chip and connectors embedded in a plastic frame of varying size.
SIM cards have evolved over the years, it is safe to say that at the moment, regardless whether you are using a feature phone or the latest smartphone or tablet, that there are different SIM cards that you can choose from, and depending on the manufacturer of your phone, you will need to get the correct-sized SIM card in order to insert it properly into your device.
Commonly Found/Used SIM Cards
Mini SIM: This SIM card has been around for many years now and can be found on older smartphones or certain feature phones even today. The size of the SIM card is pretty big and if phones like the iPhone 3GS and Nexus 4 are some examples of the phones that relied on the mini SIM card.
Micro SIM: The micro SIM is pretty much the standard these days for the majority of smartphones. Pretty much all Android smartphones at the moment accept the micro SIM as the SIM card of choice. It is noticeably smaller than the mini SIM and has helped to contribute to the creation of thinner smartphones. Phones such as the iPhone 4, iPhone 4S, Nexus 5, and Samsung Galaxy S5 are examples of phones that use the micro SIM.
Nano SIM: The nano SIM is currently the smallest SIM card available at the moment. At one point in time, there was a bit of contention between Apple and Nokia regarding the nano SIM in which Apple was looking to make the nano SIM the new standard in SIM cards.
The nano SIM is much smaller than the micro SIM and it pretty much leaves just the chip of the SIM card exposed, where the micro and full-sized SIM cards still had a bit of plastic surrounding the edge.
Apple has since adopted the nano SIM as the standard on its mobile devices starting with the iPhone 5, and Nokia has since followed suit despite their initial reluctance with phones like the Nokia Lumia 1520 and Nokia Lumia Icon, just to name a few. Android OEMs such as HTC have also started to use the nano SIM with the HTC One M8.
Embedded SIM: This is another type of SIM although its use is aimed more at M2M applications rather than consumer products like smartphones or tablets.
At one point in time it was rumored that Apple had considered the idea of incorporating embedded SIMs into its products in which customers could simply choose their carrier at an Apple store and have it activated there and then.
Usage of embedded SIMs can be found in vehicles where carriers such as Vodafone have supplied embedded SIMs to the upcoming 2015 Audi and Volkswagen models.
Rare/No Longer In Use: the full-size SIM
Safe to say that based on the design, phones were pretty thick as they had to accommodate the size and thickness of the SIM card as well as its components. You can also learn about the history of SIM cards if you are really that curious.
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