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A SIM card is an integrated circuit that securely stores the data used to identify a subscriber within a mobile network.

Originally the size of a credit card, soon the miniSIM was introduced and now is called the "standard size". The full-size SIM is no longer used with recent devices -- instead two even smaller variants have been introduced: microSIM and (only used by Apple) the nanoSIM.

SIM-Card evolution
SIM-Card evolution: full size, miniSIM, microSIM, nanoSIM (Source: Wikipedia)

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