once frame member 1 a is nested in mounting rail 70 it cannot inadvertently escape however this geometry also prevents insertion of frame member 1 a into mounting rail 70 this difficulty is overcome by gradually thinning base 72 by tapering this down so as to create a relief shoulder 90 in an inner surface thereof the distance b from lip 78 to relief 90 is selected to be slightly greater than the length l i e b gt l of the frame member 1 a it can be seen from fig3 how in use the mounting rail 70 may be secured to a supporting surface 17 by any suitable method and wall 10 end of frame member 1 a inserted into mounting rail 70 using relief shoulder 90 by passing the length l of frame member 1 a through the gap defined by distance b until the base 4 of frame member 1 a abuts base 72 of mounting rail 70 see fig4 at which point locating tab 60 locates in channel 82 tensioning of the pliable sheet 2 retained by the frame member 1 a will urge the locating tab 60 into channel 82 ensuring that the frame member 1 a is retained in the mounting rail 70 throughout the specification and the claims that follow unless the context requires otherwise the words u201c comprise u201d and u201c include u201d and variations such as u201c comprising u201d and u201c including u201d will be understood to imply the inclusion of a stated integer or group of integers.