# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 3 Ambiguities in Specifications--New Challenges  
In Figure 6, the flatness tolerance with the CZ modifier (combined zone) for two planes is shown, and, in Figure 7, the meaning of this requirement, which was already established in ISO 1101:2004, is illustrated (according to the terminology used at that time, when the CZ modifier was decoded as a _common zone_). Since then, this standard has been revised two times, but only the name of the modifier has been updated, so its meaning has not been changed. In ISO 1101:2017, it is stated that when the CZ modifier is indicated in the tolerance indicator, all the related individual tolerance zones shall be constrained in location and orientation amongst themselves using either explicit or implicit theoretically exact dimensions. Unfortunately, this statement is not univocal. The question is what recalls the constraints, in other words, what forces the restriction that, according to Figure 6, the established tolerance zone pattern is composed of two tolerances zones, which are the spaces between pairs of two parallel planes 0.06 mm apart that are constrained in orientation to be parallel (the implicit TED = 0\({}^{\circ}\)) and in location to be coplanar (the implicit TED = 0 mm). The flatness tolerance belongs to the group of form tolerances [8]. Therefore, the flatness requirement indicated by the flatness symbol does not introduce any constraints between the tolerance zones. The drawing user shall additionally identify, from the drawing of this part, the 0 mm distance between the surfaces for which the flatness tolerance is specified, but this distance is not recalled itself by the flatness tolerance symbol that completely defines the requirement. Figure 5: (**a**) Specification—the four tolerance zones, two cylindrical tolerance zones and two spaces between two parallel planes, are locked among themselves and with respect to the specified datum system. One pattern of tolerance zones is established by two combined tolerance zones. (**b**) The actual part that fulfils the specification from (**a**). Derived median lines of the top (1) and bottom (2) holes and derived median surfaces of the left (3) and right (4) grooves are in the tolerance zones. Two cylindrical tolerance zones and two tolerance zones established by two pairs of two parallel plains are locked together as one pattern. In Figure 8, according to the provisions given in ISO 1101:2017 and ISO 14405-2:2018 [15], the position tolerance without a datum is indicated to set acceptable deviations for the relative location of two nominally planar surfaces. The position tolerance belongs to the group of location tolerances [8]. Therefore, such an indication recalls the theoretically exact dimension, TED = 12 mm, which is given explicitly in the drawing. This theoretically exact dimension sets a distance of 12 mm between the median planes of the two tolerance zones (Figure 9). After comparing the indications given in Figures 6 and 8, it seems evident that to keep the consistency and general approach in the specification meanings, the position tolerance shall be indicated instead of the flatness tolerance shown in Figure 6. Only the application of the position tolerance (Figure 10) recalls the theoretically exact dimension, TED = 0 mm, which is implicitly given in the drawing. Thus, the specification given in Figure 6 is definitely wrong, even ridiculous, because the flatness tolerance does not set any constraints between the tolerance zones. So the functional meaning shown in Figure 7 is not secured by the indication given in Figure 6, which is currently subjectively overinterpreted. Similarly, according to the current industry indication and interpretation practice, it is assumed that the parallelism tolerance with the CZ modifier (Figure 11) constrains two tolerance zones for both surfaces in orientation to be parallel with respect to datum A (the implicit TED = 0\({}^{\circ}\)) as well as in location to be coplanar (the implicit TED = 0 mm). Datum A locks the part down on a surface that simulates datum plane A.