is interfitted within the track 73 formed between the opposed l shaped arms 70 and 72 of the base track 38 as shown in fig1 a base track 38 a wall panel 24 and a cross beam channel 40 extend vertically upwardly from their connection to a horizontally extending lower base track not shown the opposite lateral vertically disposed edges of the wall panel 24 are interfitted within the central track 73 and squared c channel 80 formed by the vertically disposed base track 38 and cross beam channel 40 a flange 90 positions and secures the lower end of the cross beam channel 40 relative to the lower base track further the cross beam channel 40 positions associated window framing 28 as shown in fig1 a corner post 34 connects the vertical edges of the left and front walls 18 and 20 the corner post 34 extends vertically upwardly from the substrate 12 and is connected to two base tracks 38 a the base tracks extending horizontally along the substrate and at right angles to one another the two base tracks 38 a are connected to the substrate 12 by l shaped flanges 90 in a manner described above one l shaped flange 90 is disposed in one base track 38 a and has a vertical leg 90 a received in the squared c channel 56 and threadably fastened to the wall 48 of the corner post.