- Formwork dummy as a kit for forming, on a formwork face, the blockout of an opening in a wall formed from a setting material, said blockout having the shape of a rectangular parallelepiped, wherein the dummy includes: - four filling assemblies (E1; E2; E3; E4), referred to as corner assemblies, each capable of being placed in a zone corresponding to a corner of the blockout, and in such relative positions that their outer envelope corresponds to said blockout and that there are provided, in this arrangement, a space separating facing vertical faces of the corner assemblies and a space separating facing horizontal faces of the corner assemblies, a corner assembly (E1; E2; E3; E4) being formed by a single block referred to as a "filling block" or by several filling blocks placed side by side; - removable members (3; 215) referred to as "formworking/form stripping keys" capable of being arranged in the aforementioned spaces and of holding the corner assemblies (E1; E2; E3; E4) in the corners of the blockout to be formed, said members being capable of adjusting the spacing, which depends on the dimension of said blockout, of the corner assemblies facing each other in the mounted position, and also being capable of being removed when the dummy is dismantled, the removable members being capable of holding the corner assemblies (E1; E2E2; E3; E4) in the corners of the blockout to be formed by applying, to the block or blocks of the corner assemblies, a force directed towards the periphery of the formwork dummy, - a set of closing plates capable of closing the ends of the aforementioned spaces in the mounted position to constitute, with the outer walls of the corner assemblies, the periphery of the dummy to be formed, characterised in that: each formworking/form stripping key includes two plates intended to be applied to the respective walls of two assemblies facing each other in the mounted position of the dummy, the two plates being connected by a mechanism for adjusting their spacing, wherein said adjustment mechanism may consist of: - two threaded rods each mounted in a threaded hole provided in a respective plate, the threaded rods being in line with each other, perpendicular to the plates, each carrying a bevel gear at its opposite end relative to the plate, the two bevel gears cooperating with a third gear connected to a drive for moving the plates towards or away from each other; or - racks carried by the respective plates, the racks being arranged perpendicular to the plates and cooperating with a central toothed wheel carrying a drive for moving the plates towards or away from each other; or - a threaded rod carried by one of the plates, a hollow cylindrical tube without internal thread carried by the other plate and receiving said threaded rod, and an adjustment member formed by a hollow body including an axial bore with a first threaded part cooperating with said threaded rod and a second non-threaded part of larger diameter and capable of receiving said cylindrical tube, the adjustment being carried out by manual pivoting said hollow body. - four filling assemblies (E1; E2; E3; E4), referred to as corner assemblies, each capable of being placed in a zone corresponding to a corner of the blockout, and in such relative positions that their outer envelope corresponds to said blockout and that there are provided, in this arrangement, a space separating facing vertical faces of the corner assemblies and a space separating facing horizontal faces of the corner assemblies, a corner assembly (E1; E2; E3; E4) being formed by a single block referred to as a "filling block" or by several filling blocks placed side by side; - removable members (3; 215) referred to as "formworking/form stripping keys" capable of being arranged in the aforementioned spaces and of holding the corner assemblies (E1; E2; E3; E4) in the corners of the blockout to be formed, said members being capable of adjusting the spacing, which depends on the dimension of said blockout, of the corner assemblies facing each other in the mounted position, and also being capable of being removed when the dummy is dismantled, the removable members being capable of holding the corner assemblies (E1; E2E2; E3; E4) in the corners of the blockout to be formed by applying, to the block or blocks of the corner assemblies, a force directed towards the periphery of the formwork dummy, - a set of closing plates capable of closing the ends of the aforementioned spaces in the mounted position to constitute, with the outer walls of the corner assemblies, the periphery of the dummy to be formed, - two threaded rods each mounted in a threaded hole provided in a respective plate, the threaded rods being in line with each other, perpendicular to the plates, each carrying a bevel gear at its opposite end relative to the plate, the two bevel gears cooperating with a third gear connected to a drive for moving the plates towards or away from each other; or - racks carried by the respective plates, the racks being arranged perpendicular to the plates and cooperating with a central toothed wheel carrying a drive for moving the plates towards or away from each other; or - a threaded rod carried by one of the plates, a hollow cylindrical tube without internal thread carried by the other plate and receiving said threaded rod, and an adjustment member formed by a hollow body including an axial bore with a first threaded part cooperating with said threaded rod and a second non-threaded part of larger diameter and capable of receiving said cylindrical tube, the adjustment being carried out by manual pivoting said hollow body.