normal state the distance between the open surfaces 2220 of the two hooks 22 is larger than the distance of the two stopping boss 126 the first fastener 1 is engaged with the second fastener 2 the stopping bosses 126 of the locking piece 12 are inserted into a space between the open surfaces 2220 of the hook 22 and the post 24 the second fastener 2 is made of plastic materials the distance between the two hooks 22 of the second fastener 2 is larger than the distance between the inner walls of the left side wall 102 and the right side wall 103 of the socket 10 when the second fastener 2 is inserted into the first fastener 1 the two hooks 22 of the second fastener 2 are pressed inwardly by the inner walls of the left side wall 102 and the right side wall 103 to generate an elastic deformation as a result the distance between the open surfaces 2220 of the two hooks 22 is compressed to be smaller than the distance between the two stopping bosses 126 the post 24 enters the guiding grooves 105 and moves along the guiding grooves 105 until the second locking surfaces 2222 of the two hooks 22 of the second fastener 2 abut the end surfaces of the stopping bosses 126 respectively at this time the second fastener 2 pushes the locking piece 12 to move along the guiding grooves 105 the locking piece 12 moves to press the spring.