A boss for securing a pair of mainboards includes a circular top and a plurality of first flexible legs. The first flexible legs are disposed on one side of the circular top and are separated from each other. Each of the first flexible legs includes a tilted wall and a pressing component. The tilted wall is formed on a surface of the first flexible leg that faces another first flexible leg. The pressing component is formed on one end of the first flexible leg away from the circular top. The tilted walls of all the first flexible legs cooperatively define a tapered channel. The tapered channel can be expanded.