front side of protective flap 24 is substantially parallel to the front side of window casing 8 fig1 shows furthermore that a first or lower coating 46 of plaster is applied to intrados 4 and a second or upper coating 48 of plaster is applied on top of first coating 46 in smoothing the upper coating 48 it is possible to use the outside of the left hand web 32 in fig1 as smoothing edge after the entire plastering work has been finished protective film 42 is removed and protective flap 24 is broken off at a break off material bridge 44 of reduced material thickness for reasons of simplification of the drawings fig2 4 only depict the front portion 22 of a fillet 18 omitting also a protective flap 24 fig2 illustrates an embodiment in which a reinforcing fabric section 50 is attached to fillet 18 by means of a closure member 52 closure member 52 which may consist of plastics material has been inserted at a later time through the 34 opening 34 54 of fillet 18 in the form of a slot between the free ends of the two webs 32 and is locked there in a snap fit the end portion of a reinforcing fabric section 50 in doing so has been clamped between closure member 52 and at least.