It's really, REALLY simple.

And once you know, you'll get why mason jars are so amazing!

All modern mason jars are either 'regular mouth' (the smaller of the two sizes) or 'wide mouth' (the larger of the two sizes)

You can change the volume of the jar and the function of the lid in infinite combinations knowing that everything labeled regular mouth will fit together, and everything labeled wide mouth will fit together.

Yes there are a few vintage exceptions to this rule, and commercial jars are a whole different world entirely, for details on exceptions click here.