Seven years after he wrote a book about impeachment, then-U.S. Supreme Court Chief Justice William Rehnquist presided over one: the trial of President Bill Clinton in 1999, which resulted in acquittal. The book, and the experience, made Rehnquist an unparalleled expert on impeachment.

As the U.S. House moves forward with its impeachment inquiry of President Donald Trump, one of Rehnquist’s former clerks, now-Chief Justice John Roberts Jr., would be poised to preside over any trial in the U.S. Senate. Of course, only if the process gets to that point.