Base cruck trusses and arch brace trusses are used to create a wider roof than can be bridged with a tie-beam. In the former two cruck blades are embedded in the walls of the barn with a collar beam connecting them about half-way up, as in Dunster Tithe Barn (below).

Arch brace trusses, as in Bradford-on-Avon Tithe Barn (right) are made up of two principal rafters connected with a collar. Underneath the collar there are two curved full braces jointed into the collar and rafters. This creates an arch shape below the collar beams. The two terms can be  interchangeable.

Arch Braced & Raised Cruck Trusses