Coupling/uncoupling method, plastic coupler and envelop, bag and binder

A plastic fastener comprising a first segment produced by extrusion molding a long first segment such that a first base and a protrusion formed on a front surface of the first base and having a stopper hook on its end have identical sections in one direction and by cutting the long first segment into pieces with a specified length, and a second segment produced by extrusion molding a lone second segment such that a second base and a groove formed on a front surface of the second base to nip the stopper hook of the protrusion have identical sections in one direction and by cutting the long second segment into pieces with a specified length, wherein:

• the protrusion and the groove are engaged with each other by pressures applied from respective back sides of the first base and second base;
• and
• the engagement of the protrusion and the groove is canceled only by sliding the protrusion together with the first base in the direction where the protrusion has identical sections and by sliding the groove together with the second base in the direction where the groove has identical sections.