2:1 double wire is a 
traditional book binding, 
various colours, 
firm and solid, 
360 degree reading
  A4 = 23 holes
.white.. silver
black  white  red  blue  silver 
No.  Size               Binding
                         sheets  pc
4     1/4    inch  ( 6.4 mm)     45  100  
5     5/16  inch  ( 8  mm)       60  100  
6      3/8   inch  ( 9.5 mm)     75  100  
7      7/16 inch  (11  mm)      90  100  
8       1/2  inch  (12.7 mm)  105  100  
9      9/16 inch  (14.3 mm)  120    50  
10      5/8 inch  (16 mm)     135    50  
12      3/4 inch  (19 mm)     160    50  
14      7/8 inch  (22 mm)     190    50 
16      1    inch  (25.4 mm)   220   50   

18   1-1/8 inch (28.5mm)    250   25
20   1-1/4 inch  (32 mm)     280   25   
other sizes and colours are available