While a Senate trial of President Donald Trump now appears inevitable, details of how it will unfold remain unknown. How long will proceedings last? Can either party summon witnesses to make its points? Senators will have to decide these and other, potentially thorny questions.
Presiding will be Chief Justice John Roberts — that's in the Constitution — and theoretically he could issue key rulings on some of these questions. But Roberts is not likely to want to be in the spotlight, and a Senate majority could overrule him in any case.
Almost everything can be negotiated between Republicans and Democrats.