are filled in, leaving a fairly uniform undercoat. In three coat plastering it is standard to apply a second layer in the same fashion, leaving about of rough, sandy plaster (called a brown coat or browning (UK)). A smooth, white finish coat goes on last. After the plaster is completely dry, the walls are ready to be painted. In this article's photo ("lath seen from the back...") the curls of plaster are called keys and are necessary to keep the plaster on the lath. Traditional lime based mortar/plaster often incorporates horsehair which reinforces the plasterwork, thereby helping to prevent the keys from breaking away. Historical transition In addition to wood lath, various types of metal lath began to be used toward the end of the 19th century. Metal lath is categorized according to weight, type of ribbing, and whether the lath is galvanized or not. Metal lathing was spaced across a center, attached by tie wires using lathers' nippers. Sometimes, the mesh was dimpled to be self-furring. In use as early as 1900, rock lath (also known as "button board," "plaster board" or "gypsum-board lath"), is a type of gypsum wall board (essentially an early form of drywall) with holes spaced regularly to provide a 'key' for wet plaster. Rock lath was typically produced in sheets sized . The purpose of the four-foot length is so that the sheet of lath exactly spans three interstud voids (overlapping half a stud at each end of a four-stud sequence in standard construction), the studs themselves being spaced apart on center (United States building code standard measurements). By the late 1930s, rock lath was the primary method used in residential plastering. Lath and plaster methods have mostly been replaced with modern drywall or plasterboard, which is inferior, but is faster and less expensive to install. Drywall possesses poor sound dampening qualities and can be easily damaged by moisture. Traditional lime based plasters are resistant to moisture and provide excellent sound isolation. Advantages One continued advantage of using traditional lath is for ornamental or unusual shapes. For instance, building a rounded wall would be difficult if drywall were used exclusively, as drywall is not flexible enough to allow tight radii. Wire mesh, often used for exterior stucco, is also found in combination or replacement of lath and plaster which serves similar purpose. Traditional lath and plaster (including rock and metal lath varieties) has superior