Closure for wire loop binder

A booklet binding system for use with a loop binder of the type having a pair of opposed spaced apart rows of loop segments, wherein one of the rows of segments forms narrow binding tine elements adapted to be received through the holes of pre-punched sheets and the other row of segments forms connecting loop segments spaced from the binding tine elements to provide a continuous wire loop unit,

• a closure unit adapted to be removably mounted in binding position in the space between the two rows of opposed loop segments to capture the sheets mounted on the tine elements,
• said closure unit including a central body portion and a pair of flanges defining a pair of spaced apart loop receiving grooves for respectively receiving the ends of the connecting loop segments on one side and the ends of the binding tine elements on the other side,
• means for retaining the closure unit in binding position between the loop segments,
• wherein the material of the closure unit on one side of at least one of the loop receiving grooves has different flexibility from the material on the other side of said one groove and is resiliently yieldable to permit sufficient deformation in response to pressure from the engaged row of spaced apart loop segments to resist movement of the closure unit when in assembled binding position with the loop binder.