A castor known as a "reverse locking castor" for movements over ground (So), comprising: • a wheel casing, • means for mounting said casing on a piece of furniture along a mounting axis which is substantially perpendicular to said ground (So), • at least one wheel having a tread, said wheel having an overall radius with a value Rr, and • means for mounting said wheel in rotation on a shaft defining an axis of rotation and in a manner such that said wheel is capable of making contact with the ground at at least one point of contact of said tread, this point of contact being located in a plane known as the "median plane " which contains said axis of rotation and which is substantially parallel to said mounting axis, • a housing produced in said casing, said housing being in the shape of a hollow sector of a cylinder the cylinder axis of which being substantially parallel to said axis of rotation, said housing additionally being bordered by two end walls (32, 33) forming abutments, these two end walls (32, 33) defining an angle at the centre with a value Log the vertex of which being situated on said cylinder axis, said housing comprising an opening which is opposite the ground on which the wheel is capable of rolling, said opening being defined over an angle at the centre the vertex of which is situated on said cylinder axis and having a specific value Oac corresponding to the sum of two angles at the centre with values Oac1 and Oac2 the vertices of which are situated on said cylinder axis, these two angles at the centre being contiguous along said median plane, characterized in that it further comprises • a part which is in the shape of a solid cylinder sector defined over an angle at the centre with a value Pac the vertex of which is situated on said cylinder axis, the overall radius of this part having a value Rp which is at most equal to said value Rr, • a protuberance produced on the edge of the part which faces the opening, said protuberance being defined over an angle at the centre the vertex of which is situated on said cylinder axis and the value of which is much smaller than the value Pac of the angle at the centre of the part, said protuberance comprising a vertex which is at a distance from said cylinder axis by the value Re and defined in a plane passing through said cylinder axis, known as a "protuberance plane ", said protuberance plane separating the angle at the centre with value Pac into two angles at the centre with respective values Pac1 and Pac2 separated by said protuberance plane, and • means for mounting said part on the housing so that it is capable of moving in said housing by pivoting substantially freely about said cylinder axis in order to take up two extreme positions (P1, P2), a first position when it abuts against one of the two end walls (32, 33) and a second position when it abuts against the other of the two end walls. • a wheel casing, • means for mounting said casing on a piece of furniture along a mounting axis which is substantially perpendicular to said ground (So), • at least one wheel having a tread, said wheel having an overall radius with a value Rr, and • means for mounting said wheel in rotation on a shaft defining an axis of rotation and in a manner such that said wheel is capable of making contact with the ground at at least one point of contact of said tread, this point of contact being located in a plane known as the "median plane " which contains said axis of rotation and which is substantially parallel to said mounting axis, • a housing produced in said casing, said housing being in the shape of a hollow sector of a cylinder the cylinder axis of which being substantially parallel to said axis of rotation, said housing additionally being bordered by two end walls (32, 33) forming abutments, these two end walls (32, 33) defining an angle at the centre with a value Log the vertex of which being situated on said cylinder axis, said housing comprising an opening which is opposite the ground on which the wheel is capable of rolling, said opening being defined over an angle at the centre the vertex of which is situated on said cylinder axis and having a specific value Oac corresponding to the sum of two angles at the centre with values Oac1 and Oac2 the vertices of which are situated on said cylinder axis, these two angles at the centre being contiguous along said median plane, • a part which is in the shape of a solid cylinder sector defined over an angle at the centre with a value Pac the vertex of which is situated on said cylinder axis, the overall radius of this part having a value Rp which is at most equal to said value Rr, • a protuberance produced on the edge of the part which faces the opening, said protuberance being defined over an angle at the centre the vertex of which is situated on said cylinder axis and the value of which is much smaller than the value Pac of the angle at the centre of the part, said protuberance comprising a vertex which is at a distance from said cylinder axis by the value Re and defined in a plane passing through said cylinder axis, known as a "protuberance plane ", said protuberance plane separating the angle at the centre with value Pac into two angles at the centre with respective values Pac1 and Pac2 separated by said protuberance plane, and • means for mounting said part on the housing so that it is capable of moving in said housing by pivoting substantially freely about said cylinder axis in order to take up two extreme positions (P1, P2), a first position when it abuts against one of the two end walls (32, 33) and a second position when it abuts against the other of the two end walls.