In timber framing each timber is given a name based on its location in the frame.
A tie beam usually ties two walls together going from post to post, but they can go from plate to plate. A collar beam usually goes between two rafters.
Tests by Ed Levin have shown that collars only "tie" rafters together when they are as low as one foot above the foot of the rafter, or lower.
Once someone suggested three different names for "collars" based on their elevation between the rafters. I don't recall all three names but I usually call and collar a collar beam and not a collar tie, as the word tie implies it is tying the two rafters together, when in a lot of cases it is in compression holding the two rafters from sagging.
Jim Rogers
