The big media event of the week — well, as far as we know as of now — is Tuesday night’s presidential debate. Fox News’ Chris Wallace will moderate the first of three scheduled presidential debates. (There also will be one vice presidential debate.)

So what is the role of a moderator? To ask good questions on a variety of topics and to keep the candidates focused on those topics. A good moderator also needs to make sure the candidates answer the questions asked if they try to pivot or duck.

But is it their role to fact-check? No, according to Frank Fahrenkopf, co-chair for the Commission on Presidential Debates. On his CNN “Reliable Sources” show, Brian Stelter asked Fahrenkopf if Wallace would be empowered to fact-check Donald Trump and Joe Biden.

“When we choose moderators, we make very clear that there’s a vast difference between being a moderator in a debate and being a reporter who is interviewing someone,” Fahrenkopf said. “When you’re interviewing someone, if they say something that is in direct opposition to something they said a week ago, your duty is to follow up and say, ‘Wait a minute, you didn’t say that a week ago.’ But that’s not the case in a debate.”