The principle of a typical symmetry check is illustrated in  Figure 2-87 . The precise figures, tolerances, and checkpoints  for a particular aircraft are found in the applicable service or  maintenance manual.  On small aircraft, the measurements between points are usually  taken using a steel tape. When measuring long distances, it is  suggested that a spring scale be used with the tape to obtain  equal tension. A five-pound pull is usually sufficient.  On large aircraft, the positions at which the dimensions are  to be taken are usually chalked on the floor. This is done by  suspending a plumb bob from the checkpoints and marking  the floor immediately under the point of each plumb bob.  The measurements are then taken between the centers of  each marking. When it has been determined that the aircraft is symmetrical  and structural alignment is within specifications, the cable  tension and control surface travel can be checked. To  determine the amount of tension on a cable, a tensiometer  is used. When properly maintained, a tensiometer is 98  percent accurate. Cable tension is determined by measuring  the amount of force needed to make an offset in the cable  between two hardened steel blocks called anvils. A riser or  plunger is pressed against the cable to form the offset. Several  manufacturers make a variety of tensiometers, each type  designed for different kinds of cable, cable sizes, and cable  tensions. One type of tensiometer is illustrated in  Figure 2-88 . Figure 2-88.  Cable tensiometer and sample conversion chart.  Cable Tension Symmetry Check Figure 2-87.  Typical measurements used to check aircraft symmetry. 0 20 40 60 80 100 120 No. 1  Riser  No. 2  No. 3 Diameter 1/16 3/32 1/8 12 19 25 31 36 41 46 51 16 23 30 36 42 48 54 60 21 29 36 43 50 57 63 69 30 40 50 60 70 80 90 100 110 120 12 17 22 26 30 34 38 42 46 50 20 26 32 37 42 47 52 56 60 64 5/32 3/16 7/32 1/4 Tension (lb) Trigger Pointer lock Anvil Anvil Riser Example 2-51