=37. High-Grade Locks.=—There are a number of high-grade door locks and latches in the market for the equipment of first-class buildings, and frequently these locks are especially constructed to meet particular requirements. Among such locks are included, besides high-grade, three-tumbler locks, those that embody the cylinder principle and those that are equipped as master-key locks. [Illustration: FIG. 41] The =unit-cylinder lock=, illustrated in Fig. 41, is made by P. & F. Corbin. This hardware specialty is a new departure in lock making. The mechanism of the lock is contracted into the smallest possible space, occupying only about 1½ in. × 3¼ in. Instead of being mortised into the stile of the door, as is usual with the mortise lock, a piece is cut entirely from the stile, as illustrated in Fig. 42. In order to prevent this operation from weakening the stile of the door, the unit-cylinder lock is provided with heavy escutcheon plates that are strongly ribbed on the back, so that when these plates are secured to the stile at the top and bottom, they supply the rigidity necessary to make up for the notching. As the name implies, these locks are made in a unit, and the keyhole to the cylinder lock is located in the knob. The unit-cylinder lock is made in two styles—with dead-locking latch bolt and with additional dead bolt. [Illustration: FIG. 42] =38. High-Grade Interior Door Locks.=—In Fig. 43 is shown a type of high-grade, =mortise-locking latch= for interior doors. These locks are of the heaviest construction, and are fitted with either brass or bronze fronts. The bolts are operated with two or more tumblers constructed on an improved pattern, and are of fine workmanship. The trim for a lock of this character should be of the very best grade, with knobs of the screwless-spindle type, which will be explained later. The escutcheons should be of the high-collar or the bracket-bearing type, and should conform to the character of the locks in quality and finish. On the most important work, it is always expedient and usually feasible to obtain expert advice from manufacturers or from dealers representing the manufacturer, whose intimate knowledge of the product will be of great assistance to the architect or owner in making suitable selections. The use of high-grade hardware requires a considerable expenditure, and the necessity for expert advice consequently becomes more imperative if the best results are to be obtained. [Illustration: FIG. 43] =39. Master-Key Locks.=—The type of lock known as the =master-key lock= is generally used for public or office buildings, hotels, and occasionally in the better class of residence work. These locks can be grouped into two classes; namely, the _Yale_, or _cylinder_, and the _lever-tumbler types_, the class first mentioned being the most desirable.