Cross bracing for Profile 1 - Round Edge and Profile 3 - Sharp Edge Stretcher Bars. 

For added strength and statbility on larger sized frames, a set of Cross Braces (B and C) is required for stretcher bars that are 48" and above; that extend over in two directions and up to 84" x 84".

You must have one brace type B and one one brace type C for a Cross Brace.