For the 12 years of his mayoralty, Michael R. Bloomberg dominated New York City's political landscape to the extent that he seemed to be a fixture, almost an inevitability. His era began in the traumatic months after the Sept. 11 terrorist attacks, when the city sought to recover from the destruction of the World Trade Center. It continued through recovery, prosperity, financial collapse, social protests and lingering controversy over his own third term in office. A younger generation of voters can remember no previous mayor. When Bill de Blasio enters City Hall as mayor on Jan. 1, he will be the first Democrat to do so in two decades. Here, Times reporters and columnists look back at the Bloomberg legacy in some of the key areas that most affect New Yorkers.