BACKGROUND OF THE INVENTION 
     This invention relates to a drywall trim piece for installing at an external right angle corner at which two drywall boards meet and which is provided with a bullnose corner bead. 
     In standard wall board construction practice, an external corner joint, formed where wall board of a first wall surface meets wall board of a second wall surface, may be covered with a protective corner bead. The corner bead is typically made of steel, formed as a single strip held in place against the wall boards of the two wall surfaces by nails or by a thin layer of joint compound between the flanges of the corner bead and the wall surfaces. When the corner bead has been secured in position, joint compound is applied over the flanges of the corner bead and the adjacent wall surfaces to provide a smooth finish. 
     After the drywall has been installed and finished, baseboard is typically installed along the base of each wall. At external corners, the two strips of baseboard material that meet at the corner are typically mitered to provide a uniform external surface profile up to the point at which the strips meet. 
     The conventional method of installing corner bead and baseboard is advantageous because it allows fairly wide tolerance on positioning the lower end of the corner bead. As long as the lower end of the corner bead is below the upper edge of the base board, there is no unsightly gap or rough edge. Since the base board is typically at least three inches wide, it is not necessary to cut the corner bead very accurately. 
     This technique for installation of drywall and baseboard has worked well when the corner bead has a substantially right angle profile because the exterior configuration of the corner bead, after the joint compound has been applied, matches the interior configuration of the baseboard at the corner. Recently, however, bullnose corner bead, in which the corner bead includes, between the planar flanges, an intermediate portion having a radius of curvature as large as one inch, has come into favor. If conventional 450° miters are used in the baseboard material at an external corner provided with bullnose corner bead, there can be an unsightly gap between the exterior of the bullnose corner bead and the strips of baseboard material. 
     Several techniques have been developed for avoiding the problem created by the difference between the external configuration of the drywall corner and the internal configuration of the baseboard. One technique involves use of a trim piece which provides a transition between the curved exterior surface of the bullnose corner bead and the right angle internal corner of the baseboard. This trim piece is made of a hard synthetic polymer material. At its upper end, the trim piece has a tongue which is shaped and sized to fit under the bullnose corner bead. Just below the tongue, the exterior surface of the trim piece substantially matches the exterior surface of the corner bead. There is then a transition area, about ½ inch high, over which the configuration of the exterior surface changes from one that matches the exterior surface of the corner bead to a right angle, which matches the internal configuration of the baseboard corner. The corner bead is installed at the corner so that its lower end is slightly above the upper edge of the baseboard that is to be used. The trim piece is installed with its lower end resting on the floor and the tongue at its upper end is inserted under the lower end of the bullnose corner bead. The baseboard is then installed and the trim piece provides a smooth transition from the external configuration of the corner bead to the internal configuration of the base board corner. 
     In practice, the known trim piece is subject to several disadvantages. For example, the lower end of the corner bead must be accurately positioned to within about ¼ inch above the upper edge of the baseboard material. Also, the height of the lower end of the corner bead determines the height of the baseboard and it is costly and inconvenient to use baseboard material of different height. Further, the height of the trim piece depends on the height of the baseboard material and therefore an installer who is working with several different heights of baseboard material must hold several different sizes of trim pieces in inventory, increasing the cost of inventory and giving rise to the danger that the wrong size pieces will be shipped to a particular job site. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided a trim piece for installing at a drywall external right angle corner at which two drywall boards meet and which is provided with a bullnose corner bead having first and second flanges extending over the two drywall boards respectively and a convexly curved portion joining the first and second flanges, the first and second flanges having respective mutually parallel free edges, said trim piece having first and second ends and comprising an intermediate portion which wraps over the convexly curved portion of the bullnose corner bead and has first and second edges extending parallel to the edges of the flanges of the corner bead and also has first and second ends, first and second leaves attached to the intermediate portion of the trim piece at the first and second edges respectively, and first and second pins projecting from the first and second leaves respectively, the first and second pins being located so that when the trim piece is placed over the corner bead and the leaves are forced against the flanges of the corner bead, the pins are driven into the drywall boards and a flank of each pin bears firmly against a free edge of a flange of the corner bead, whereby the trim piece is held in position relative to the corner bead, and the intermediate portion of the trim piece having an interior surface at said first end matching closely the exterior surface of the corner bead at said first end and an exterior surface which is convexly curved at said first end and is right-angled at said second end and provides a transition from the convex curve to the right angle between said first and second ends. 
     In accordance with a second aspect of the invention there is provided a trim piece for installing at a drywall external right angle corner at which two drywall boards meet and which is provided with a bullnose corner bead having first and second flanges extending over the two drywall boards respectively and a convexly curved portion joining the first and second flanges, the first and second flanges having respective mutually parallel free edges, said trim piece having first and second ends and comprising an intermediate portion which wraps over the convexly curved portion of the bullnose corner bead and has first and second edges extending parallel to the edges of the flanges of the corner bead and also has first and second ends, and first and second leaves attached to the intermediate portion of the trim piece at the first and second edges respectively, and the intermediate portion of the trim piece having an interior surface at said first end matching closely the exterior surface of the corner bead at said first end and an exterior surface which is convexly curved at said first end and is right-angled at said second end and provides a transition from the convex curve to the right angle between said first and second ends. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which 
     FIG. 1 is a horizontal sectional view of an external right angle corner provided with bullnose corner bead and a first trim piece in accordance with the invention, 
     FIG. 2 is an interior perspective view of the trim piece, 
     FIG. 3 is an exterior perspective view of the trim-piece, 
     FIG. 4 is a perspective view of the trim piece installed at an external right angle corner, and 
     FIG. 5 is a view similar to FIG. 2 of a second trim piece in accordance with the invention. 
    
    
     In the several figures of the drawings, the thicknesses of various elements are exaggerated for the sake of clarity. 
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an external corner of a structure including a vertical stud  10  and drywall boards  12  attached to the stud by nails (not shown) and meeting at substantially a right angle to form an external corner  18 . A strip  22  of bullnose corner bead has a curved intermediate portion  26  and two planar flanges  28  which meet the intermediate portion  26  at respective steps  34 . The corner bead is positioned so that the two flanges  28  lie against the external surfaces of the drywall boards  12  respectively. The corner bead is held in position by nails (not shown) or it may alternatively be secured to the drywall boards by a thin layer of joint compound (not shown) between the flanges and the drywall boards. 
     FIGS. 2 and 3 illustrate a trim piece in accordance with the invention. The trim piece is made of synthetic polymer material such as ABS, which is quite hard. The trim piece has a body  40 . An optional gauge strip  44 , which is shown in phantom, is attached to the body  40 . The function of the gauge strip will be described below. 
     The body  40  of the trim piece is composed of an intermediate portion  48  and two flanges  52 . Two tabs  56  are attached to the flanges  52  at respective edges thereof. The thickness of the polymer material is such that the trim piece is stiff except along the lines  60  at which the tabs are attached to the body, where the material is sufficiently thin to act as a hinge and allow pivotal movement of the tabs  56  relative to the body  40  of the trim piece. 
     The two tabs  56  are each provided with upper and lower pins  62 . The trim piece is made by injection molding and the mold is designed so that in the unstressed condition of the trim piece, the two tabs are bent outwardly from the planar surfaces of the flanges  52 , such that the tips of the pins extend only slightly, if at all, beyond the planes of the flanges  52 . 
     The height of the body  40  is less than the minimum conventional height of standard baseboard material, which is sold in widths of from about two inches to about eight inches. 
     The two flanges  52  of the body  40  have flat interior surfaces which meet the interior surface of the intermediate portion at steps  64  which match the steps  34  of the corner bead. The flanges  52  are uniform in configuration over the height of the trim piece. 
     Over the greater part of its height, the intermediate portion  48  of the body  40  of the trim piece is uniform in cross-section, having flat external surfaces that meet substantially at a right angle. 
     At the upper end of the trim piece, the internal surface  68  of the intermediate portion is concavely curved to fit closely against the convexly curved external surface of the intermediate portion  26  of the corner bead and the external surface of the intermediate portion is convexly curved. Slightly below the upper edge of the trim piece, the external surface of the intermediate portion is right angled, and between the upper edge and the slightly lower right angle is a transition area  66  which provides a smooth transition of the external surface from the convexly curved configuration at the upper edge to the right angle configuration. 
     The trim piece is installed by positioning it so that the bottom of the transition area  66  is at the expected level of the upper edge of the baseboard and accordingly the upper end of the trim piece is slightly above the upper edge of the baseboard. The trim piece is positioned so that the intermediate portion  48  sits firmly against the corner bead. The concavely curved surface  68  at the upper end of the trim piece fits snugly against the convexly curved portion of the corner bead, the flanges  52  of the trim piece fit against the flanges  28  of the corner bead, and the steps  64  between the flanges  52  and the intermediate portion  48  of the body of the trim piece fit in the steps  34  between the flanges  28  and the intermediate portion  26  of the corner bead. In this position, the tips of the pins bear only lightly if at all against the drywall boards. By virtue of the steps  64  of the trim piece fitting in the steps  34  of the corner bead, the trim piece is reliably held in alignment with the corner bead. 
     While holding the trim piece in position, as described, the installer strikes the two tabs  52  with a hammer, driving the pins into the drywall. The pins are positioned so that when they are driven fully into the drywall, and the tabs are in contact with the drywall over substantially their entire area, the inner flank of each pin is in firm contact with the edge of the flange  28  of the corner bead. The positions of the pins are selected so that as the pins are driven fully into the drywall board, the steel corner bead causes slight deformation of the pins. The result of this action is that the body of the trim piece is under tension between the tabs  56  and accordingly the pins are held in the fully inserted position by frictional engagement between the flanks of the pins and the free edges of the flanges and/or by virtue of the free edges of the flanges digging into the flanks of the pins. It will be noted that the inner flanks of the pins  62  are nearly perpendicular to the inner surfaces of the tabs  56  in order to maximize frictional engagement of the flanges  28  with the pins. 
     When the trim piece has been installed, as described above, joint compound is applied in conventional fashion over the flanges of the corner bead and over the flanges  52  and tabs  56  of the trim piece, down at least to the top edge of the baseboard. 
     Finally, after the joint compound has cured, the baseboard is installed. Since the trim piece was placed so that the upper edge of the baseboard would be substantially at the bottom of the transition area, the transition area spans the gap between the external surface of the corner bead and the interior of the corner at which the baseboard strips meet. 
     The gauge strip  44 , which extends parallel to the edges of the intermediate portion, is perforated at intervals of ½ inch so that it can readily be broken, allowing the installer to easily position the trim piece at the appropriate height for a particular standard width of baseboard material. 
     The trim piece shown in FIG. 5 is similar to that shown in FIGS. 1 and 2 except that there are no tabs  56  but the flanges  52  instead extend beyond the free edges of the flanges  28  of the corner bead and the pins  62  project from the flanges  52 . In use, the installer places the trim piece so that the flanks of one pair of pins  62  rests firmly against the free edge of one of the flanges  28  of the corner bead and strikes the flange  52  lightly to drive the pins into the wall board. At this point, the flanges  52  and the intermediate portion  48  may be slightly flexed. The installer positions the other pair of pins  62  so that their tips are in contact with the wall board and strikes the flange  52  lightly with a hammer, driving the pins into the wall board. The trim piece is then seated snugly against the corner bead. As in the case of the trim piece described with reference to FIGS. 2 and 3, the pins are held in the fully inserted position by frictional engagement between the flanks of the pins and the free edges of the flanges  28  and/or by virtue of the free edges of the flanges  28  digging into the flanks of the pins. 
     It will be appreciated that the invention is not restricted to the particular embodiments that have been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof. For example, although the trim piece described with reference to the drawings is made of synthetic polymer material, it may be made of another suitable material such as sheet metal, which can readily be formed to the desired shape. Further, the trim piece may be held in position by a mechanism other than the pins  62 , such as nails. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.
A trim piece for installing at a drywall external right angle corner which is provided with a bullnose corner bead includes an intermediate portion which wraps over the convexly curved portion of the bullnose corner bead and has two edges extending parallel to the edges of the flanges of the corner bead. Two leaves are attached to the intermediate portion of the trim piece at the two edges respectively. The intermediate portion of the trim piece has an interior surface at a first end matching closely the exterior surface of the corner bead and an exterior surface which is convexly curved at the first end and is right-angled at the opposite second end and provides a transition from the convex curve to the right angle between the first and second ends.