Rule #1 in geometric dimensioning and tolerancing (GD&T), often referred to as the Taylor Principle, establishes a fundamental relationship between size and form for features of size. It states that a feature must have perfect form at Maximum Material Condition (MMC). This requirement is not optional; it is an inherent rule of GD&T unless explicitly overridden.
To understand Rule #1, it is helpful to begin with the definition of a feature of size. A feature of size is any feature that can be measured across opposing surfaces, such as the diameter of a hole or the width of a slot. These features are typically controlled using size limits, for example:
2.30 ± 0.10
This establishes an allowable range for the feature, but Rule #1 adds an important condition: at the most restrictive size, the feature must also be geometrically perfect.
For an internal feature such as a hole, MMC corresponds to the smallest allowable diameter. At this condition, the feature contains the greatest amount of material and therefore presents the most restrictive case for assembly. Rule #1 requires that the hole be perfectly round and free from form error at this size. Any deviation from perfect form would effectively reduce the available space and could interfere with a mating component.
As the feature departs from MMC toward Least Material Condition (LMC), additional form variation becomes permissible. This is because the feature is gaining clearance. For a hole, as the diameter increases, small imperfections in roundness or cylindricity can be tolerated without affecting function, provided that all points of the surface remain within the specified limits of size.
This relationship can be understood by imagining two ideal boundaries:
The inner boundary defined by the MMC size
The outer boundary defined by the LMC size
The actual surface of the feature must lie entirely between these two limits. When the feature is at MMC, both boundaries coincide, leaving no room for form error. As the feature grows toward LMC, the allowable region between these boundaries increases, permitting greater variation in form.
Rule #1 is often interpreted physically through the use of functional gaging. For a hole, a fixed gage pin corresponding to the MMC size can be used to verify compliance. If the feature is truly at MMC, the pin must fit perfectly without interference, which implicitly requires perfect form. As the feature becomes larger, the same gage will continue to fit even if the surface is slightly irregular, because additional clearance is available.
An important implication of Rule #1 is that size limits alone impose a degree of form control. Even in the absence of an explicit geometric tolerance, a feature cannot violate perfect form at MMC. This built-in control is why GD&T drawings do not always include separate form tolerances for every feature.
However, Rule #1 does not guarantee acceptable form under all conditions. When a feature departs from MMC, the allowable form variation increases, which may not be desirable in all applications. If tighter control of shape is required regardless of size, additional geometric tolerances such as roundness, cylindricity, or profile must be specified.
Understanding Rule #1 is essential because it bridges the gap between size tolerancing and geometric control. It ensures that features function properly at their most critical condition while allowing reasonable manufacturing flexibility as conditions become less restrictive. In practice, it provides a balance between precision and producibility, which is central to effective engineering design.