triple sewing supports

Preferred label: 

A triple support has three elements lying side by side and, in the single example recorded on a late 15th-century German binding, was made from alum-tawed strap-type supports with two knife-cut across the spine.
Sewing supports which consist of three elements of a suitable material lying side by side across the spine of a bookblock.

Broader concept: 
Broader partitive concept: 
Top concept: