Key limes (also known as bartender limes, West Indian limes, or Mexican limes) are smaller than regular limes and have higher acidity (especially when freshly picked). The more ripe the key limes, the less tart and more sweet they become. Key limes are bright green when they are first picked but as they ripen, they become more yellow. You can decide how tart or sweet your pie is by choosing limes based on their ripeness.