Despite what many people think, key limes and regular limes are not the same thing. There are some very noticeable differences between key limes and “Persian limes”, which are the limes you usually see being sold at the grocery store. Persian limes are usually larger, thicker-skinned, less bitter, and much greener than key limes.
Key limes are usually a lot smaller than regular limes with a thin skin that are sold green for the most part, though they are a bright yellow color when they are ripe. Key limes are also typically much more acidic than Persian limes and have more of a bitter tart flavor than their counterparts.