Unless you're speaking of a specific one, it's conventions, people, not convention. As in the Geneva Conventions, more than one. We can argue about whether waterboarding violates a Geneva Convention, but not the Geneva Convention. But there's no need to argue at all because, you know, the people we currently torture are, conveniently, not prisoners of war.
As long as I'm in the middle of a pedantic fit, nothing is comprised of anything. One thing comprises other things. A jury comprises twelve people. A six-pack comprises six bottles. Okay, you can put away your crayons and get ready for recess now.