The sides of the fireplace should be vertical up to the throat, or damper opening (_ff_ fig. 34). The throat should be 6 to 8 inches or more above the bottom of the lintel and have an area not less than that of the flue and a length equal to the width of the fireplace opening. Starting 5 inches above the throat, _ee_, the sides should be drawn in at _tt_ to equal the flue area. Proper throat construction is so necessary to a successful fireplace that the work should be inspected several times a day during construction to make certain that the side walls are carried up perpendicularly until the throat is passed and that the full length of opening is provided. Smoke Shelf and Chamber The smoke shelf is made by setting the brickwork back at the top of the throat to the line of the flue wall for the full length of the throat. Its depth may vary from 6 to 12 inches or more, depending on the depth, d, of the fireplace. [Illustration: Figure 37.--Grates of this type are commonly used in fireplaces for burning coal or coke. This one has a metal back and ends and is only set in to permit proper circulation of air around it.] The smoke chamber is the space extending from the top of the throat, _ee_, up to the bottom of the flue proper, tt 9 and between the side walls. The walls should be drawn inward 30° to the vertical after the top of the throat, _ee_, is passed and smoothly plastered with cement mortar not less than one-half inch thick. Damper A properly designed damper, as shown in figure 34, affords a means of regulating the draft and prevents excessive loss of heat from the room when the fire is out. A damper consists of a cast-iron frame with a lid hinged so that the width of the throat opening may be varied from a closed to a wide-open position. Various patterns are on the market, some designed to support the masonry over the opening, others requiring lintel irons. [Illustration: Figure 38.--This well-designed small stone fireplace was built in accordance with the principles given in this bulletin. It is a good heater and does not smoke. The jambs are wide enough to resist the thrust of the arch.] A roaring pine fire may require a full-throat opening, but slow-burning hardwood logs may need only 1 or 2 inches of opening. Regulating the opening according to the kind of fire prevents waste of heat up the chimney. Closing the damper in summer keeps flies, mosquitoes, and other insects from entering the house down the chimney. In houses heated by furnaces or other modern systems, lack of a damper in the fireplace flue may interfere with uniform heating, particularly in very cold windy weather, whether or not there is a fire on the hearth. When air heated by the furnace is carried up the chimney there is a waste of the furnace fuel, but a damper partially open serves a slow fire of hardwood without smoking the room or wasting heated air from the main heating system.