# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 3 Ambiguities in Specifications--New Challenges
It is important to emphasise that parallelism tolerance belongs to the group of orientation tolerances [8]. Therefore, the parallelism requirement indicated by the parallelism symbol does not introduce any location constraints between the tolerance zones. The drawing user shall additionally identify, from this part drawing, the 0 mm distance between the surfaces for which the parallelism tolerance is specified, but this distance is not itself recalled by the parallelism tolerance symbol that shall completely define the requirement. To express in the univocal way the functional requirement illustrated in Figure 12, that both surfaces are fixed among themselves, the position tolerance shall be indicated. Such a specification enforces the implicit theoretically exact dimension, TED = 0 mm. The parallelism requirement for both toleranced surfaces with respect to datum A shall be defined using the modifier \(>\)\(<\) (for the orientation constraint only [8]) after datum identifier A in the tolerance indicator's third section (Figure 13). Figure 11: The parallelism tolerance with the CZ modifier: two toleranced surfaces shall be considered together—widely used but not fully defining specification. Figure 9: Two extracted surfaces shall be simultaneously contained between two pairs of two parallel planes with a distance of 0.05 mm and with a distance of 12 mm between their median planes. Another questionable example of pattern specification taken from ISO 5458:2018 [10] is given in Figure 14. In Figure 15, the current meaning of this specification is shown. The tolerance feature is a collection of two extracted median lines. The tolerance zone is a tolerance pattern composed of two cylindrical zones with a diameter of 0.1 mm, where their axes are constrained in location to be coaxial. Such an interpretation is not justified by the specification given in Figure 14. The straightness tolerance belongs to the group of form tolerances [8]. Therefore, the straightness requirement indicated by the straightness symbol does not set any constraints between the tolerance zones. The drawing user shall additionally identify, from this part drawing, the 0 mm distance between the axes of two holes for which the straightness tolerance is specified, but this distance application is not itself recalled by the straightness tolerance symbol that shall completely define the requirement. The conclusion is that the position tolerance shall be indicated to specify the functional requirement depicted in Figure 15. Only the application of the position tolerance, as shown in Figure 16, recalls the theoretically exact dimension, TED = 0 mm, implicitly given by the narrow dashed-dotted symmetry line. It is worth adding that the functional requirement depicted in Figure 15 may also be uniquely specified, as shown in Figure 17. The coaxiality tolerance for each hole with respect to the common data A-B is established by the specification in Figure 17. The coaxiality tolerance locates each cylindrical tolerance zone on the datum axis by the implicit theoretically exact distance, TED = 0 mm. So the two tolerance zones have the same axis that composes the tolerance zone pattern, as is required. The actual explanation is a bit more sophisticated due to the usage of a common datum (Figure 18). Figure 12: Two extracted surfaces shall be simultaneously contained between two pairs of two parallel planes with a distance of 0.04 mm and parallel to datum A—intended functional meaning.