Mini-Bookler® Ring Segments, Round 5″ (12.7cm). The Mini-Bookler Segmented Rings allow you to change the size (and shape) of rings. This 5″ radius curved segment is the smaller of the curved Mini-Bookler segments. In virtually every case, they are used in pairs. Using a pair by themselves makes a round ring. With straight segments included, they become oval rings. All Mini-Bookler Rings and Segments feature notches for use with disposable hook retractors.