Many points of cutting have been covered-chiefly those concerned with fabric stability and train. Fabrics must always be cut in a relaxed flat, a perfectly straight state, and if a fabric is difficult to be maintained in this manner, it should be stabilized by contact with paper or another fabric. Sharp scissors, or knives, are necessary for all cutting and synthetic fibers because these are tough and need sharp cutting edges. Often where high speed cutting knives are used by clothing manufactures constant sharpening maintenance is necessary and lubrication to prevent too much heat developing and fusing layers of fabrics together. Sewing primarily involves attention to several points, needle size, thread type, size and tension, and stitch frequency is the most important. These vary according to the type of fabric being sewn. The needle must pierce the fabric rapidly and accurately without damaging it. If a needle persistently cuts or breaks continuous filament yarns as the fabric is sewn, a line of fiber ends sticking up like bristles will be visible down the seam and spoil the smoothness. Fabric made from synthetic filaments are particularly susceptible to cutting so that needles with a fine cloth or ball point should be used and fines needle size should be used. Different manufacturers have different methods of needle sizing but the normal singer range of sizes 10 to 12 to continental sizes 70 to 80, are common suitable needle sizes for synthetic fiber fabrics. The choice of a suitable thickness and type of sewing thread is important but as a general rule the finest thread size consistent with adequate strength be selected since the majority of seams or at least the stitches in them are required to be as unobtrusive as possible. Cotton covered polyester sewing threads are acceptable for most purposes but for synthetic fiber fabrics and where particularly high strength and resistance to shrinkage is concerned synthetic fiber fabrics sewing threads are preferable. Tension of thread in needle and spool and needle and spool should be as low as is consistent with a balanced stitch, i.e. the crossing or spool and needle thread should take place in the fabric and not either on the back or in the surface. Synthetic threads are very strong and elastic and if fed into the machine at too great a tension they can actually draw the seam together in a pucker when the tension relaxes. Correct seaming requires control of the stitch interval. This varies from about 15 to 19 or 22 stitches per 3cm. It is desirable that a seam should have the same lengthwise amount of give as the fabric otherwise it will have binding effect and will be subjected which could cause it to snap. Seams for extensible fabrics should contain more stitches to the cm than seams for close and firm fabrics because they have more elasticity. Similarly seams for light and open fabrics should be close in order to hold the fabric securely and prevent yarn slippage. Where knitted fabrics are being sewn seams, which are necessary for garment stability, are usually taped or supported by binding. With firmly woven fabrics curved seams usually require frequent notching of the inlay to give the required suppleness and evenness of curve. Neatening of seams is essential in the case of filament yarn fabrics and loosely woven staple fiber fabrics, particularly those of rayon, to prevent untidiness through unraveling of fabric. The overlook machine is used extensively by clothing manufacturers for this purpose as well as for the general sewing of knitted fabrics.