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Greg has already correctly answered Jason's question, but I want to add a comment on the matters that were confusing Jason. Yes, it is true that one King may never check or capture another King, but this is true only as a consequence of the rules, not as a rule itself. The relevant rules are that neighboring Kings are in check from each other, and a King may never move into check. It follows from these two rules that one King may never move next to another, and that is what stops one King from checking or capturing another. In other words, Kings have the ability to check each other, but the possession of this ability keeps them from ever having the opportunity to use it. The statement that Kings may never check each other is true from an absence of opportunity, not from an absence of ability.