CombBind Standard SpinesCommunicate your ideas with professional polish. 19-ring plastic spines won't scratch, chip, peel or discolor. Bind 11 x 8 1/2 documents. Editing capability with a machine. Compatible with GBC and other plastic comb binding machines. To select correct diameter, measure thickness of material (including covers) and add 1/8". Sheet Capacity: 90; Spine Diameter: 1/2"; Number of Rings: 19.