The design of a masonry wall to support vertical compression loading is influenced by the same factors that affect the design of all compression elements. One of these factors is the extent of rotational restraint at the top of the wall. Explain how and why the rotational restraint at the top of a loaded wall is different if the load is due to a concrete slab which is sitting on the wall, compared to if the loads are being applied by, say, timber trusses which are sitting on the wall.