For a larger library these rails must be made into partitions, giving to each department a separate room. Partitions of glass set in wooden frames and possibly only eight feet high may answer an excellent purpose, adding to the impression of extent, admitting light to the interior of the building and allowing some supervision from the center. With partitions on each side, the entrance becomes a central hallway with a department at each side and the book room at the end. This is the best position for the book room for two special reasons. Overlapping the departments in both wings it is equally accessible from either, and at the back of the house a plainer and cheaper wall can be built admitting of easy removal when the growth of the library requires enlargement. Sometimes the angles between the book room and the main building may be filled to advantage by work room and office. These working rooms though not large and not conspicuous are of vital consequence and should be carefully planned. We have now reached a type of building which, for lack of a better word, I may call the "butterfly plan," having two spread wings and a body extending to the back. Others call it the "trefoil." This general type is being substantially followed in most new libraries of moderate size. From one entrance hall direct access is given to three distinct departments, or perhaps to five, by placing two rooms in each wing. _Modifications required by limited space._ If we have an open park to build in we shall be tempted to expand the hallway to a great central court or rotunda. Perhaps the importance of the library may justify it, but we should be on our guard against separating departments by spaces so great as to make supervision difficult or passing from one to another inconvenient. We should aim to concentrate rather than scatter. More frequently the lot will be too narrow. We must draw in the wings and make the narrower rooms longer from front to back. With a corner lot we can enter on the side street, leaving a grand reading room on the main front and turning at right angles as we enter the house pass between other rooms to the book room at the extreme end of the lot. Or again, we shall be obliged to dispense entirely with one wing of our plan, and have but two department rooms instead of three on the floor. Every location must be studied by itself. _Other stories._ Basement rooms are of great service for work rooms and storage. A basement directly under the main book room is specially valuable to receive the overflow of books not in great demand. A second and even a third story will be useful for special collections, class and lecture rooms or a large audience hall. In a library of moderate size it will often be found convenient to build a book room about 16 feet high to cover two stories of bookcases and wholly independent of the level of the second floor of the main building.