I think it is helpful to refer to a piece in a frame depending on it's use. I also think it can be important to identify pieces designed to be in compression vs. tension since it can change the joinery (and engineering?). Ties are in tension. If a piece is not in tension, it should not be called a tie. Just to make things controversial, king-posts and queen-posts are designed to be in tension and thus should not be called posts!
Jim
