The back-band which is fit for no other use but to bear up the side ropes, should, if you tramel all four legs, be made of fine girth-web, and lined with cotton; but if you tramel but one side, then a common tape will serve, taking care that it carries the side ropes in an even line, without either rising or falling: for if it rises it shortens the side-rope, and if it falls there is danger of its entangling.
Source: wiktionary