The county of Allegheny comprises the heart of the Pittsburgh district as usually understood, and includes the city itself, termed by the laws of the commonwealth a city of the second class; three cities of the third class, i.e., McKeesport, Clairton, and Duquesne; sixty-six boroughs and fifty-four townships (of which twenty-five are townships of the first class, governed by a board of commissioners, and twenty-nine are townships of the second class, under the control of a board of supervisors). During the years 1925–26, the boroughs of Carrick, Knoxville, and Westwood and the township of Lower St. Clair have, by the voluntary act of their people, united with the city of Pittsburgh. There are now existing within the county of Allegheny a total of one hundred twenty-four separate municipal units (exclusive of school districts), each operating under its own particular form of government.