Warp indicates the longitudinal fibers of a fabric. The warp  is the high strength direction due to the straightness of the  fibers. A warp clock is used to describe direction of fibers  on a diagram, spec sheet, or manufacturer’s sheets. If the  warp clock is not available on the fabric, the orientation is  defaulted to zero as the fabric comes off the roll. Therefore,  90° to zero is the width of the fabric across.  [Figure 7-3] All product forms generally begin with spooled unidirectional  raw fibers packaged as continuous strands. An individual fiber  is called a filament. The word strand is also used to identify  an individual glass fiber. Bundles of filaments are identified  as tows, yarns, or rovings. Fiberglass yarns are twisted, while  Kevlar$^{®}$ yarns are not. Tows and rovings do not have any  twist. Most fibers are available as dry fiber that needs to  be impregnated (impreg) with a resin before use or prepreg  materials where the resin is already applied to the fiber. Most fabric constructions offer more flexibility for layup  of complex shapes than straight unidirectional tapes offer.  Fabrics offer the option for resin impregnation either by  solution or the hot melt process. Generally, fabrics used  for structural applications use like fibers or strands of  the same weight or yield in both the warp (longitudinal)  and fill (transverse) directions. For aerospace structures,  tightly woven fabrics are usually the choice to save weight,  minimizing resin void size, and maintaining fiber orientation  during the fabrication process. Unidirectional prepreg tapes have been the standard within  the aerospace industry for many years, and the fiber is  typically impregnated with thermosetting resins. The most  common method of manufacture is to draw collimated raw  (dry) strands into the impregnation machine where hot melted  resins are combined with the strands using heat and pressure.  Tape products have high strength in the fiber direction and  virtually no strength across the fibers. The fibers are held in  place by the resin. Tapes have a higher strength than woven  fabrics.  [Figure 7-4] A roving is a single grouping of filament or fiber ends, such  as 20-end or 60-end glass rovings. All filaments are in the  same direction and they are not twisted. Carbon rovings are  usually identified as 3K, 6K, or 12K rovings, K meaning  1,000 filaments. Most applications for roving products utilize  mandrels for filament winding and then resin cure to final  configuration.  Unidirectional (Tape)  Warp Clock Roving  Figure 7-1.  Bidirectional and unidirectional material properties. Figure 7-3.  A warp clock. Bidirectional (Fabric) Fiber Forms 7-3 Bidirectional Unidirectional Equal properties Unequal properties 0 0 90 90 +45 −45 +45 −45 90° 0° 0° 0° 90° +45° –45° 90° +45° –45° Figure 7-2.  Quasi-isotropic material layup.