A SIM card (SIM = Subscriber Identification Module) is a physical chip card with which users can identify themselves in the network of a mobile communications provider. The SIM card is inserted into a special card slot on the mobile device and activated.
In many smartphone models, the SIM module is already permanently installed; here, registration can be carried out solely by means of a digital code; the process is then called eSIM ("embedded SIM").
Devices that are designed to accept and use two SIM cards are called dual-SIM models. SIM cards are available in various sizes. In most cases, they are provided by the mobile network provider as triple SIM cards. Here, all three common sizes (mini-SIM, micro-SIM, nano-SIM) are supplied on a credit-card-sized full-size SIM (the fourth, but now uncommon size).
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