It would have seemed routine in many places: A defendant accused of illegally possessing a gun sat across a gleaming courtroom from the judge who accepted his guilty plea and would pronounce his sentence.

For Mexico, though, it was a remarkable change from a century-old judicial system of paper-shuffling court cases in which defendants rarely actually testified before the judge ruling on their fate from within a cramped, bureaucrat’s office. As of Saturday, the open, oral trial will be the norm nationwide as part of a sweeping judicial reform.