Stacking normally refers to an option you can select for Uninsured Motorist Bodily Injury (UM) and/or Underinsured Motorist Bodily Injury (UNDUM) coverages.
Stacking Uninsured / Underinsured Motorist policies is an option that allows you to increase the limits you select for your UI/UIM bodily injury coverage. Limits increase based on the number of cars you are insuring. Keep in mind this increased level of protection typically will raise your insurance premium.
An example of stacking is John has limits of $100,000/$300,000 for his uninsured motorist bodily injury coverage and is insuring 2 vehicles. If he leaves them "unstacked," or non-stacked as you called it, John's limits would stay at $100,000/$300,000. If John instead chooses to "stack" his UMBI coverages then his limits would double to $200,000/$600,000.
So by selecting stacking for your uninsured/underinsured motorist bodily injury coverage, you increase your limits for each of these coverages by the number of cars you're insuring. Generally there is not a limit on the number of vehicles that can be insured and thus stacked this way.
[Let CarInsurance.com help you find affordable auto insurance now.] To the best of our knowledge there are currently nineteen (19) states whose statutes, rules, and/or case law either do not address the issue or specifically allow it. However, in many states that allow stacking, auto insurers are permitted to include policy language that prevents policyholders from stacking UM/UIM coverage. So while your state might permit stacking, if your policy explicitly forbids it, you will not be able to stack your benefits.
The 19 states are: Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Hawaii, Iowa, Kentucky, Maine, Michigan, Mississippi, Missouri, Nevada, Oklahoma, Pennsylvania, Rhode Island, Vermont and West Virginia.
Since laws in each state vary widely for UM/UIM BI stacking in accordance with each situation and sometimes depending upon case law, (and state insurance laws are continually being changed and update), it is best to check with your state's insurance regulator as well as your insurance agent to find out if you can stack your UM/UIM benefits.
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Since "stacking" is the application of more than one policy limit to the same loss or occurrence, if you only have one vehicle on your policy than there would not be coverage to stack. With a single car policy there is not multi-coverage to stack thus no benefit since this option would not be available to you.
Click here to get free auto insurance quotes and see if your state allows for stacking on your UM/UIM coverages. At CarInsurance.com the coverage page of our quote form will give you the option of choosing stacked coverage if it is available. | 
