The SIM card is the heart of your smartphone. Without it you would not be able to make calls, send texts or connect to a data service to browse the web and send emails.

There are several different SIM card sizes on the market and while they all serve the same purpose, it's worth noting that different handsets have different sized SIM-slots.

When you get a mobile phone as part of a pay-monthly contract deal or a pay-as-you-go package, the right SIM card will be included as part of the bundle. If you're happy with your current phone, you may want to order a SIM-only deal. Luckily, when you order a SIM through mobiles.co.uk, you'll receive a Multi-SIM which can be snapped down to fit any sized SIM slot.

Standard SIM/ Mini SIM

Plenty of people will be familiar with Mini SIM cards, which were the first stage in the miniaturisation of this type of item and are still used widely in feature phones and some smartphones today. Measuring 25mm by 15mm, these will slot under the battery or into the side of smartphones, so check the individual manual of your handset to work out where to find the SIM compartment.

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Micro SIM

Enter the Micro SIM, which Apple debuted way back with the iPhone 4. It has the same chip as a standard SIM, but takes up less space and will therefore only fit into a compatible slot. This is the middle size of the three SIM sizes available, which means there's another SIM which is even smaller...

As smartphones have become thinner, it was necessary for smaller SIM cards to be created.

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Nano SIM

This is a thinner, smaller SIM card that allows manufacturers to make their handsets even sleeker, as evidenced by Appleā€™s flagship mobiles ever since the iPhone 6.

This is the smallest SIM size currently made, and therefore is the innermost selection when snapping down your Milti-SIM.

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