Probably a stupid question and it's likely been asked before but the search wasn't too helpful.

When building a part that's likely to go into an assembly, is there a way to reference a dimension on the second part to a dimension on the first part?  For example, if you're building a two-piece part, where a tab on part one inserts into a slot on part two, and a screw fastens them together.  What's the proper way to make it so that the length of the tab corresponds to the depth of the slot, but still have them two separate files should they be required to be used on a different assembly?

The way I've been doing it is to jump back and forth between files and check the dimensions to make sure they correspond.  I know there's got to be an easier way, and I have no doubt that I already know how to do it but don't remember how due to not using it in the past 5 years.

Thanks in advance for any help