How do areas with multiple MLS work?

[Henley H.]

I've heard of areas where there are multiple MLS systems within a single county. I understand they're each independently owned and am curious how it works when multiple MLS systems exist within one county- Do they each only cover a portion of the county, do you need a membership to each one to have access to all listings within that county, etc.? Any clarity on this would be appreciated- thanks!