The 12th district consists of portions of both Allegheny and Westmoreland Counties. A portion of Allegheny County consists of the cities of Clairton, Duquesne, McKeesport and Pittsburgh and the townships of Elizabeth, Forward, North Versailles, South Park, South Versailles, Upper St. Clair and Wilkins and the boroughs of Baldwin, Bethel Park,

Braddock, Brentwood, Bridgeville, Chalfant, Dravosburg, East McKeesport, East Pittsburgh,

Elizabeth, Glassport, Homestead, Jefferson Hills, Liberty, Lincoln, Monroeville, Mount Oliver, Munhall, North Braddock, Pitcairn, Pleasant Hills, Plum, PortVue, Rankin, Swissvale (PART, Districts 01, portions of 02, Trafford (Allegheny County Portion), Turtle Creek, Versailles, Wall, West Elizabeth, West Homestead, West Mifflin, Whitaker, White Oak, Whitehall and Wilmerding. The district also includes parts Westmoreland County consisting of the city of

Jeannette and the townships of Hempfield and districts East Adamsburg, High Park, Lincoln Heights West, parts of  Middletown, Wegley, Wendel Herm and West Hempfield), North Huntingdon, Penn, Sewickley and South Huntingdon (PART, District Yukon) and the boroughs of Adamsburg, Arona, Export, Irwin, Madison, Manor, Murrysville, North Irwin, Penn, Sutersville  and Trafford (Westmoreland County Portion).