A method and apparatus is disclosed for mounting a rail support post to the perimeter of a stairway having stingers at the edge of the stairway and treads, risers and possibly blocking laterally across the stairway. The apparatus includes a bracket member that is attached to or against the interior of a stringer and includes at least one hole for accommodating a fastener that passes through the stringer and into or through a post member at the outside surface of the stringer. The bracket includes flanges that attach to a rise or a tread and/or a block to secure the assembly together.