=69. Sash Centers.=—When a transom sash is hung at the top or the bottom, regular hinge butts may be used; but where a sash is pivoted at the center, either on the sides or at the top and bottom, a pivoted arrangement, termed a =sash center=, is needed. For large sash or heavy transoms, and especially for those that are exterior sash, the rabbeted center should be used. This type of center is illustrated in Fig. 84; its construction gives great strength and completely closes the joint against light and water. An excellent type of center, known as the _Howarth sash center_, is illustrated in Fig. 85, the method by which it is attached to the frame and sash being clearly indicated. By this arrangement, the two parts of the center fold, or butt, against each other and form a tight joint, the stop-bead for the sash at the top and bottom being arranged as indicated at _a a_. [Illustration: FIG. 85] There is a sash center, known as the _Tabor_, that is constructed as shown in Fig. 86. This center is used for very large sash that are vertically pivoted; that is, arranged with a center at the top and bottom. Such sash as these are used extensively in office buildings, because, by their use, the entire window may be thrown open at one time. The Tabor device consists of a special sub-sill, which engages with a ribbed joint strip placed on the bottom of the sash. By this means, a weather-tight joint is secured, and the sash is firmly locked in a closed position. By throwing a lever, the sash is raised above the astragal, or sub-sill, and can then be operated. The top rail is supplied with a filling strip having an irregular joint at the intersection with the sash, and is held firmly by a coil spring encircling the top pivot. In Fig. 87 are shown several sash centers of the common type. These possess no particular merit, but are much used for common work. [Illustration: FIG. 86] [Illustration: FIG. 87]