What do rivets join together

pieces of metal

Generally it's used to hold metal together when welding and soldering isn't an option, but it can be used to connect many other things. I use it to connect plastic to/or metal to/or leather. In some situations glue would work too, but rivets are decorative.

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Aluminum or steel sheet metal, typically. One can, of course, join other types of objects.

Any solid parts.