referring to fig1 - 3 , a coupling mechanism for interfacing between a wiper and a wiper arm is provided in the instant disclosure , comprising an connector 1 and at least one width - adjusting spacer member 2 , where the connector 1 includes a connecting mechanism ( not shown ) for connection to a wiper . since the connective structure between the connector 1 and the wiper is known to those who are skilled in the instant field , therefore no further illustrations upon this shall be provided herein . for the instant disclosure , preferably , the connector 1 is made of plastic , furthermore , the connector 1 has a body 11 , where the two side walls of the body 11 are defined as the outer side walls 111 . the body 11 has a stopping portion 12 formed at one end thereof , where the width and height of the stopping portion 12 are larger than that of the body 11 to prevent the wiper arm 3 from sliding into the body 11 in lengthways . the bottom surface of the body 11 has two protrusions 13 respectively formed thereon where the protrusions 13 protrude from the two sides of the body 11 . for the instant embodiment , the protrusion 13 is further connected to the stopping portion 12 for the structure to be strengthened . specifically speaking , the protrusions 13 can support the bottom surface of the wiper arm 3 . to provide further explanations , the body 11 has an engaging member 14 formed at the top surface thereof . the engaging member 14 is partially connected to the body 11 by forming two troughs 141 respectively at the two sides thereof the engaging member 14 is an elastic arm which has flexibility to perform elastic movements . preferably , the elastic engaging member 14 is formed at one end of the body 11 , away from the stopping portion 12 . for the instant embodiment , the amount of the width - adjusting spacer member 2 can be one , two , or many , where this is not restricted thereto , and the width - adjusting spacer member 2 can be connected either directly or indirectly to the connector 1 . for the instant embodiment , there are two width - adjusting spacer members 2 for example . the two width - adjusting spacer members 2 are connected respectively to the connector 1 by a pivotal portion 21 to form integrally as a single unit . preferably , the width - adjusting spacer members 2 are connected to one end of the body 11 , away from the stopping portion 21 . for the instant embodiment , the width - adjusting spacer members 2 are connected directly to the body 11 , where the width - adjusting spacer members 2 can be selectively arranged on the outer side walls 111 of the body 11 . a folded line 22 can be further formed between the width - adjusting spacer member 2 and the connector 1 , where the folded line 22 is capable of separating the width - adjusting spacer member 2 from the connector 1 by tearing the folded line 22 when the width - adjusting spacer member 2 is no longer in use . preferably , the folded line 22 is formed between the pivotal portion 21 and the width - adjusting spacer member 2 . the position of the width - adjusting spacer member 2 is adjustable by pivoting the width - adjusting spacer member 2 about the pivotal portion 21 . the width - adjusting spacer members 2 will be arranged adjacently to the outer side wall 111 of the body 11 when the width - adjusting spacer member 2 pivots forwardly to a predetermined position for the total width of the coupling mechanism to increase . on the contrary , when the width - adjusting spacer members 2 pivot backwardly to a predetermined position , the total width of the coupling mechanism will be reduced . to provide further explanations , by separating the width - adjusting spacer members 2 from the connector 1 can also reduce the total width of the coupling mechanism . referring to fig4 to 5 , when the 19 mm wiper arm 3 is applied in the instant embodiment , the width - adjusting spacer member 2 have to be pivoted forwardly to a predetermined position . hence , the width - adjusting spacer members 2 will be arranged adjacently to the outer side wall 111 of the body 11 for the total width of the coupling mechanism to increase . during assembly , a front opening of the wiper arm 3 is slid along one end of the connector 1 in a favorable direction for assembly thereto , such that the wiper arm 3 is connected to the connector 1 and the outer side of the width - adjusting spacer member 2 . concurrently , the engaging member 14 of the connector 1 will be depressed . when the wiper arm 3 has slid to the predetermined position , the engaging member 14 will be in alignment with an engaging hole 31 of the wiper arm 3 , where the pressure exerted on the depressed engaging member 14 will be released for the engaging member 14 to return to the original position . hence , the engaging member 14 will clip - in into the engaging hole 31 for the connector 1 to assemble to the wiper arm 3 . furthermore , the wiper arm 3 has an inner wall 32 . after the width - adjusting spacer member 2 is arranged adjacently to the body 11 , the width - adjusting spacer member 2 will be received either partially or entirely within the inner wall 32 of the wiper arm and the outer side wall 31 of the body 11 . please refer to fig6 and 7 . when the 16 mm wiper arm is applied in the instant embodiment , the total width of the coupling mechanism can be reduced by separating the width - adjusting spacer members 2 therefrom . during assembly , a front opening of the wiper arm 3 is slid along one end of the connector 1 in a favorable direction for the wiper arm 3 to connect to the outer side of the assembly . meanwhile , the engaging member 14 of the connector 1 will be depressed . when the wiper arm 3 has slid to a predetermined position , the engaging member 14 will be in alignment with an engaging hole 31 of the wiper arm 3 , where the pressure exerted on the depressed engaging member 14 will be released for the engaging member 14 to return to the original position . hence , the engaging member 14 will clip - in into the engaging hole 31 for the connector 1 to assemble to the wiper arm 3 . referring to fig1 and 14 , the total width of the coupling mechanism can also be reduced by pivoting the width - adjusting spacer member 2 backwardly to a predetermined position . during assembly , a front opening of the wiper arm 3 is slid along one end of the connector 1 in a favorable direction for assembly thereto , where this movement will depress the engaging member 14 of the connector 1 . when the wiper arm 3 has slid to a predetermined position , the engaging hole 31 of the wiper arm 3 will be in alignment with the engaging member 14 , where the pressure exerted on the depressed engaging member 14 will be released for the engaging member 14 to return to the original position . hence , the engaging member 14 will clip - in into the engaging hole 31 for the connector 1 to assemble to the wiper arm 3 , and where the width - adjusting spacer members 2 can be received in the wiper arm 3 . referring to fig8 , according to another embodiment of the instant disclosure , the amount of the width - adjusting spacer member 2 is two for example . the two width - adjusting spacer members 2 are connected by a connecting portion 23 , where the two width - adjusting spacer members 2 are connected indirectly to the connector 1 . in other words , the two width - adjusting spacer members 2 and the connecting portion 23 cooperatively formed a substantially u - shaped structure , where the u - shaped structure can be arranged selectively on the outer side of the connector 1 . to provide further explanations , when the width - adjusting spacer members 2 are arranged on the outer side of the connector 1 , the total width of the coupling mechanism will be increased . otherwise , when the width - adjusting spacer members 2 are removed from the connector 1 , the total width of the coupling mechanism will be reduced . referring to fig9 and 10 , when the 19 mm wiper arm 3 is applied in the instant embodiment , the two width - adjusting spacer members 2 will be arranged on the outer sides of the connector 1 , such that the width - adjusting spacer members 2 are arranged adjacently to the outer side wall 111 of the body 11 for the total width of the coupling mechanism to increase . during assembly , a front opening of the wiper arm 3 is slid along one end of the connector 1 in a favorable direction for assembly thereto , where this movement will depress the engaging member 14 of the connector 1 . when the wiper arm 3 has slid to a predetermined position , the engaging hole 31 of the wiper arm 3 will be in alignment with the engaging member 14 , where the pressure exerted on the depressed engaging member 14 will be released for the engaging member 14 to return to the original position . hence , the engaging member 14 will clip - in into the engaging hole 31 for the connector 1 to assemble to the wiper arm 3 . referring to fig1 and 12 , when the 16 mm wiper arm 3 is applied in the instant embodiment , the two width - adjusting spacer members 2 can be dismantled for the total width of the coupling mechanism to reduce . during assembly , a front opening of the wiper arm 3 is slid along one end of the connector 1 in a favorable direction to assemble thereto , where this movement will depress the engaging member 14 of the connector 1 . when the wiper arm 3 has slid to a predetermined position , the engaging hole 31 of the wiper arm 3 will be in alignment with the engaging member 14 , where the pressure exerted on the depressed engaging member 14 will be released for the engaging member 14 to return to the original position . hence , the engaging member 14 will clip - in into the engaging hole 31 for the connector 1 to assemble to the wiper arm 3 . the descriptions illustrated supra set forth simply the preferred embodiments of the instant disclosure ; however , the characteristics of the instant disclosure are by no means restricted thereto . all changes , alternations , or modifications conveniently considered by those skilled in the art are deemed to be encompassed within the scope of the instant disclosure delineated by the following claims .