Windows set in the wall in the normal way do not offer many problems and most carpenters are familiar with them. Window boards should always be tongued into the sills. When a bay window extends to two storeys or roof height, the problem arises of supporting the first floor joists which must not be carried by the window head and the finish of the window at roof level. In the fixing of door frames, precision is all-important if subsequent door hanging problems are to be avoided. Doors which swing freely between jambs returning auto-matically to the closed position can be hinged to the jambs with various types of spring hinges. Sliding doors come in three main groups: that is straight sliding, round the corner, and folding. In deciding which type to use, the designer is governed by the space and position available for them when opened.