# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 2 New Rules That Reduce Specification Ambiguity  
A vital example of the ambiguity and even outright contradiction found in the GPS standards for many years is the specification of the position or symmetry tolerance for a few toleranced features. In the past, it was a common practice that the adjacent indication (indication given above the tolerance indicator) defines the number of toleranced features (Figure 1) that generates the pattern. It was also stated that when two or more groups of features are shown on the same axis, they shall be considered to be a single pattern when they are not related to a datum or are related to the same datum or datum system. There are no unique arrangements in ISO 5458:1998 regarding the position tolerance for the rectangular pattern of holes. Therefore, designers and verifiers extrapolated the rules given for the circular pattern to rectangular patterns. On the other side, according to the _independency principle_ (ISO 8015:2011 [11]), by default, every GPS specification for a feature or relation between features shall be fulfilled, independent of other specifications, except when it is stated in a standard or by a special indication added to a particular specification. Moreover, it was not clear whether the specifications given in Figures 1 and 2 are equivalent. According to ISO 5458:1998, indication 2\(\times\) in Figure 1 creates one toleranced feature, whereas two separate indications for two features are given in Figure 2. It is not clear whether the pattern of two holes is defined in Figure 2, or two independent requirements are given. For the pattern, the two extracted median lines of holes with a nominal diameter of 10 mm shall be contained in two cylindrical tolerance zones with a diameter of 0.1 mm, the axes of which are located on the plane perpendicular to datum A, which passes through datum B. Datum A is established by the tangent plane to the disc's hidden surface. Datum B is established by the axis of the cylinder inscribed into the central hole with an orientation constraint (perpendicularity), with respect to the primary datum A. Moreover, cylindrical tolerance zones are located at the theoretically exact distance of 20 mm from datum B. For the two individual indicators extracted, the median line of each hole is independently considered and shall be contained in the cylindrical tolerance zone with a diameter of 0.1 mm, the axis of which is perpendicular to datum A and contained in the half plane that starts from datum B. In the following figures, the possibilities for the unambiguous specifications of the various functional requirements regarding the relative locations of holes and grooves are shown. The rules and modifiers given in the latest edition of the revised ISO 5458, published in 2018, are used and discussed. In Figure 3a, the axis of each individually considered cylindrical tolerance zone with a diameter of 0.1 mm for the position tolerance (the SZ modifier is indicated) is perpendicular to datum A--the tangent plane to the extracted datum's surface that is at a radius of 20 mm from datum B--which is the axis of the cylinder that is inscribed into the extracted central hole with a perpendicularity constraint with respect to datum A. The symmetry planes of the two pairs of two parallel plains establishing the individual tolerance zone with a width of 0.08 mm (the SZ modifier is indicated) for each groove are perpendicular to datum A and pass through datum B. The verification of the conformance with specification shall  
Figure 1: Position tolerance with one tolerance indicator for two holes—according to ISO 5458:1998, the holes, by default, constitute the pattern. Figure 2: Position tolerance with two tolerance indicators individually given for two holes—lack of univocal meaning. be independently performed (individually) for each of the four toleranced geometrical features (Figure 3b).