Windows in all types of buildings were large and regularly placed on a grid; this was partly to minimize window tax, which was in force throughout the period in the United Kingdom. Some windows were subsequently bricked-in. Their height increasingly varied between the floors, and they increasingly began below waist-height in the main rooms, making a small balcony desirable. Before this the internal plan and function of the rooms can generally not be deduced from the outside. To open these large windows the sash window, already developed by the 1670s, became very widespread. Corridor plans became universal inside larger houses.