First things first, is there really a difference between “capital” and “capitol?”
Yes, there really is.
Okay! So what is the difference between “capital” and “capitol?”
While they might seem confusing, fear not. There is a very easy trick to navigating the difference.
First off, with just one exception, you ALWAYS spell it capital. With an “A,” capital applies to money (Capital One), cities (capital city), offenses (capital punishment) and whether a letter is upper or lower case (capital letter).
The ONLY time you use the “O” is when you are talking about a building. While it might seem silly, Washington, DC is the nation’s capital, but Congress meets in the US Capitol building. There are no exceptions to remember. You always use the “A” except for the building, when you use the “O.”
So now the trick: just picture the Capitol building’s large (and perfectly round) rotunda. It looks just like the “O” in Capitol.
Voila, you're an expert. Now wow your friends!