84-hour rule
The 84-hour rule is a College of Arts and Sciences graduation requirement that says you cannot earn more than 84 credit hours from a single division (i.e. humanities, natural science or social science). This means that at least 28 credit hours total must be from courses outside the division your major is in (and keep the 56-hour rule and the 9-9-9 distribution rule in mind).
ExCos are considered outside the three divisions of studies, so they're good for getting you up to five credits so you can graduate.
The rule doesn't apply to double degree students.
[edit] Opinions
Seriously. It's a load of bullshit, especially if you're a double major within the same division. Being well-rounded is overrated.

