A chain comprising: a plurality of inner link units each including two inner chain plates that correspond in position to each other, any two adjacent ones of said inner link units being spaced apart from each other in a revolving direction, each of said inner chain plates having two inner connecting sections and an inner waist section that is located between said inner connecting sections, said inner waist sections of each of said inner link units cooperating with each other to define an inner meshing space therebetween; a plurality of outer link units, any two adjacent ones of said outer link units being spaced apart from each other in the revolving direction, said outer link units and said inner link units being disposed in an alternating arrangement in the revolving direction, at least one of said outer link units including two outer chain plates which correspond in position to each other and between which at least one of said inner link units is located, each of said outer chain plates having two outer connecting sections and an outer waist section that is located between said outer connecting sections; and a connecting rod unit including a plurality of connecting rods each interconnecting said inner chain plates of one of said inner link units and one of said outer link units via said inner connecting sections and said outer connecting sections; whereby said outer waist section of each of said outer chain plates has a protruding part and a recessed part respectively located at two opposite sides of an imaginary central plane, and each interconnecting said outer connecting sections, and a first edge and a second edge respectively corresponding in position to said protruding part and said recessed part; said outer waist sections of said at least one outer link unit cooperate with each other to define an outer meshing space therebetween; said outer meshing space has a wide opening that is located between said first edges of said outer chain plates, and a narrow opening that is located between said second edges of said outer chain plates; characterized in that a distance between said inner waist sections of said inner chain plates of each of said inner link units is smaller than a distance between said protruding parts of said outer chain plates of said at least one of said outer link units, and is equal to or smaller than a distance between said recessed parts of said outer chain plates of said at least one of said outer link units. a plurality of inner link units each including two inner chain plates that correspond in position to each other, any two adjacent ones of said inner link units being spaced apart from each other in a revolving direction, each of said inner chain plates having two inner connecting sections and an inner waist section that is located between said inner connecting sections, said inner waist sections of each of said inner link units cooperating with each other to define an inner meshing space therebetween; a plurality of outer link units, any two adjacent ones of said outer link units being spaced apart from each other in the revolving direction, said outer link units and said inner link units being disposed in an alternating arrangement in the revolving direction, at least one of said outer link units including two outer chain plates which correspond in position to each other and between which at least one of said inner link units is located, each of said outer chain plates having two outer connecting sections and an outer waist section that is located between said outer connecting sections; and two outer connecting sections and an outer waist section that is located between said outer connecting sections; and a connecting rod unit including a plurality of connecting rods each interconnecting said inner chain plates of one of said inner link units and one of said outer link units via said inner connecting sections and said outer connecting sections; whereby said outer waist section of each of said outer chain plates has a protruding part and a recessed part respectively located at two opposite sides of an imaginary central plane, and each interconnecting said outer connecting sections, and a first edge and a second edge respectively corresponding in position to said protruding part and said recessed part; whereby said outer waist section of each of said outer chain plates has a protruding part and a recessed part respectively located at two opposite sides of an imaginary central plane, and each interconnecting said outer connecting sections, and a first edge and a second edge respectively corresponding in position to said protruding part and said recessed part; said outer waist sections of said at least one outer link unit cooperate with each other to define an outer meshing space therebetween; said outer meshing space has a wide opening that is located between said first edges of said outer chain plates, and a narrow opening that is located between said second edges of said outer chain plates; characterized in that a distance between said inner waist sections of said inner chain plates of each of said inner link units is smaller than a distance between said protruding parts of said outer chain plates of said at least one of said outer link units, and is equal to or smaller than a distance between said recessed parts of said outer chain plates of said at least one of said outer link units.