1

I have a text input field where the user enters an address. I am adding the ability to save the address to an address book. What should the icon look like to save? This is for smartphones so space is limited.

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What symbol should the plus sign be, as plus isn't always a signifier for "save to address book".

4

The Floppy disk icon is a common icon that's been engraved into the minds as the Save icon.

Apart from that, you could also use the Tick to signify Done and Save

floppy tick

Ideally you should follow the guidelines specified for the Smartphone's OS.

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2

I like Google Material Icons' Add Location icon:

Google Material Icon: Add Location

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