chapter, various fabrics and treatment systems are discussed,  as well as basic covering techniques. To facilitate the discussion of fabric coverings for aircraft,  the following definitions are presented.  Figure 3-2  illustrates  some of these items. When a fabric-covered aircraft is certificated, the aircraft  manufacturer uses materials and techniques to cover the  aircraft that are approved under the type certificate issued  for that aircraft. The same materials and techniques must be  used by maintenance personnel when replacing the aircraft  fabric. Descriptions of these materials and techniques are  in the manufacturer’s service manual. For example, aircraft  originally manufactured with cotton fabric can only be  re-covered with cotton fabric unless the Federal Aviation  Administration (FAA) approves an exception. Approved  exceptions for alternate fabric-covering materials and  procedures are common. Since polyester fabric coverings  deliver performance advantages, such as lighter weight,  longer life, additional strength, and lower cost, many older  aircraft originally manufactured with cotton fabric have  received approved alteration authority and have been re- covered with polyester fabric.  Legal Aspects of Fabric Covering Figure 3-2.  Aircraft fabric nomenclature. Fabric Terms •  Warp—the direction along the length of fabric. •  Fill or weave—the direction across the width of the  fabric. •  Count—the number of threads per inch in warp or  filling. •  Ply—the number of yarns making up a thread. •  Bias—a cut, fold, or seam made diagonally to the warp  or fill threads. •  Pinked edge—an edge which has been cut by machine  or special pinking shears in a continuous series of Vs  to prevent raveling.  •  Selvage edge—the edge of cloth, tape, or webbing  woven to prevent raveling. •  Greige—condition of polyester fabric upon completion  of the production process before being heat shrunk. •  Cross-coat—brushing or spraying where the second  coat is applied 90° to the direction the first coat was  applied. The two coats together make a single cross  coat.  [Figure 3-3] Figure 3-3.  A single cross coat is made up of two coats of paint applied 90° to each other. 3-3 First application Second application (applied when first  coat is tacky) Selvage edge Selvage edge Pinked edge Bias Fill Warp