It was a picture, unofficial but nevertheless sure to be memorialized and parsed for years, both because of its occasion and its rarity, of President-elect Donald J. Trump and his wife, Melania, seated next to President Barack Obama, who was seated next to George W. Bush and his wife, Laura, who were next to Bill and Hillary Clinton. All of them sat a row behind President Biden and Jill Biden, and all them wore remarkably similar dark suits, ties in shades of black to sky blue, and black suits and coats. (Michelle Obama was the sole presidential spouse who did not attend, reportedly because of scheduling conflicts.)

That meant that for the first time since Mr. Trump began his assault on what he called the Washington swamp, he and the other four living presidents, including the three who campaigned against him, appeared to be on the same page. Not because of a dress code, but because of their coded dress.