are fixedly attached to the corner connectors 18 by the attachment means 44 either on site or at the factory in the preferred embodiment in which the open end 32 are mitered it is important that the corner connectors 18 are plugged into the open end 32 a suitable distance to ensure a tight miter before attachment by the attachment means 44 after the wall opening 12 has been inspected to verify that the opening sizes and wall thicknesses are acceptable the frame members 48 are fitted aligned and anchored in the wall opening 12 where the assembly is for a door frame the upper horizontal casing member 14 is first attached to the horizontal frame member by pressing or popping the casing member 14 onto casing clips 56 that have previously been attached to the frame member the vertical casing members 14 which have a corner connector 18 attached to their upper open end 32 by the attachment means 44 are pulled a slight distance away from the casing clips 56 protruding from the horizontal frame members 48 and simultaneously plugged into the open end 32 of the horizontal header casing member 14 having thus described the invention it should be apparent that numerous structural modifications and.