Profile tolerances are defined in ISO 1101, and profile tolerancing is described in ISO 1660.

With profile tolerancing a distinction must be made between tolerancing of the form of lines (symbol and of the form of surfaces (symbol ).

The nominal (theoretical exact, geometrical ideal, true) form is to be defined by theoretical exact (rectangular framed) dimensions (TEDs) with (Figs 4.2 and 20.12) or without (Figs 4.1, 4.3 and 20.10) relation to datum(s). ^[*]

Figure 4.1: Profile tolerance of lines; the nominal form is defined by consecutive straight lines and circles defined by theoretical exact dimensions without reference to datums

Figure 4.2: Profile tolerance of lines; the nominal form is defined by theoretical exact dimensions with reference to datums (A-B)

The tolerance zone of a profile tolerance is defined by (tangential) envelopes on circles (profile tolerance of a line) or on spheres (profile tolerance of a surface) whose diameters are equal to the tolerance value and centred on the nominal form (Figs 4.1, 4.2 and 3.2). Therefore the zone is equally disposed on either side of the nominal profile.

In order to change the location of the tolerance zone the nominal dimensions can be recalculated or the tolerance zone can be unequally disposed relative to the nominal profile as shown in Fig. 4.3.

Figure 4.3: Unequally disposed profile tolerance zone

When the width of the tolerance zone varies within the profile see Fig. 20.10.

The form of the envelope of the circles or spheres (tolerance zone) between the specified...