Stacked car insurance lets you combine uninsured motorist (UM) coverage limits across multiple vehicles under the same policy or across multiple policies that list your name. For instance, if you have a $25,000 UM limit on one car and $50,000 on another, stacking gives you up to $75,000 in total coverage if you're injured by an uninsured driver.
There are two main types of insurance stacking: stacking within one policy and stacking across multiple policies, also known as vertical and horizontal stacking, respectively.