Assemblies are a way to test your parts you have created and make sure all the parts will work together. You can also use assemblies to make sure your parts will fit and move together.
Mates are a way to connect parts together. There are many different types of mates that you can use to connect parts together including fastened, revolute, slider, and cylindrical.
Relations are a way to connect parts together. There are many different types of relations that you can use to connect parts together including gears, rack and pinion, and screws.
This video will show you more about assemblies.
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