# Perpendicularity

Interpreting datum-related 90-degree orientation zones and measuring the required surface or derived feature.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, gdt, perpendicularity

## Meaning and specification boundary

Perpendicularity uses a linear zone oriented at 90° to a specified datum.

With a 0.05° tilt in the ideal two-line example, a straight 50 mm segment spans **0.04363 mm** normal to the intended direction and a 100 mm segment spans **0.08727 mm**. Only the first fits the illustrative 0.050 mm zone. This assumes the correct datum and a zone free to contain the segment.

KEYENCE classifies perpendicularity as a datum-related orientation tolerance, distinguishing it from datum-independent form controls. Its surface illustration uses a pair of parallel limiting planes perpendicular to the datum. This describes an enclosing zone over the controlled feature, rather than a requirement on a single convenient edge. [KEYENCE orientation tolerances](https://www.keyence.com/ss/products/measure-sys/gd-and-t/type/orientation-tolerance.jsp)

Identify the governing system and edition. ASME lists **Y14.5-2018 (R2024)**; ISO lists **ISO 1101:2017**, edition 4, February 2017, confirmed in 2022. ISO's public scope covers geometrical-specification symbols and interpretation rules. These separate frameworks should not be mixed by assuming that similar symbols imply identical defaults. [ASME record](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing/2018), [ISO 1101 record](https://www.iso.org/standard/66777.html)

## Surface and axis requirements

For an ordinary surface indication, the specified surface must fit within its oriented two-plane zone. Bow, twist and local high points can affect the result even when the fitted plane's angle is satisfactory. Keep the full controlled extent in the measurement plan.

For an axial feature, the indication may instead require a cylindrical zone or a planar zone. **PC-DMIS 2023.1** explicitly documents both shapes, and requires additional zone-orientation information for an axial planar zone. It also distinguishes acquired surface data from a derived toleranced feature. Selecting a cylinder in a measurement program is therefore not enough to establish the correct evaluation. [Hexagon perpendicularity, zone shapes and feature types](https://docs.hexagonmi.com/pcdmis/2023.1/en/helpcenter/mergedprojects/core/geometric_tolerances/Perpendicularity.htm)

| Information | Question to resolve |
| --- | --- |
| Controlled feature | Is the requirement on a surface, line or derived axial feature? |
| Zone | Is the tolerance a plane separation or a cylinder diameter? |
| Datum references | What ideal reference and precedence establish the orientation? |
| Extent and modifiers | What part of the feature and what additional conditions apply? |
| Other requirements | Where are size, location and form specified independently? |

A perpendicular orientation does not alone establish a feature's required location or size. Do not infer a hole's position from an orientation result, or interpret the diameter of its perpendicularity zone as the hole's size tolerance.

## Datum establishment and measurement

Establish the specified reference from the actual datum feature using the governing rules and suitable measurement construction. A fixture face or machine axis becomes relevant only through its relationship to that requirement. An arbitrary best fit of the entire part can answer a different question from the specified datum system.

KEYENCE illustrates a square-and-feeler-gauge arrangement and a CMM approach that first measures the datum reference. It identifies limitations of the simple square method. Treat these as possible arrangements, rather than a universal choice of instrument or point count for acceptance. [KEYENCE measuring perpendicularity](https://www.keyence.com/ss/products/measure-sys/gd-and-t/orientation-tolerance/perpendicularity.jsp)

Inspect seating, cleanliness and restraint before interpreting a change after reseating. Use coverage capable of revealing the expected form and orientation errors over the required extent. Local contact near an accessible edge may miss variation elsewhere on a wide face.

Hexagon's versioned example shows a measured result smaller than the actual value when points are insufficiently dense. Its nominal feature definitions also affect zone orientation and the available degrees of freedom. Preserve nominal definitions and evaluation settings with the routine when comparing results. [Hexagon measured-value and validity discussion](https://docs.hexagonmi.com/pcdmis/2023.1/en/helpcenter/mergedprojects/core/geometric_tolerances/Perpendicularity.htm)

## Original bounded orientation calculation

Consider a perfectly straight segment in a two-dimensional section, with true length **L = 50 mm**, tilted **δ = 0.05°** from the intended perpendicular direction. Assume a correctly established datum and no other form or setup error. Its spread normal to the intended direction is:

`spread = L × |sin δ| = 50 × sin 0.05° ≈ 0.04363 mm`.

At **L = 100 mm**, the same angular error gives **0.08727 mm**. Against an illustrative **0.050 mm** two-line zone in that section, the first ideal segment can fit and the second cannot, if the zone is otherwise free to contain the segment under the applicable constraints.

This shows why a linear orientation requirement depends on the controlled extent. It is not a universal conversion of perpendicularity to an angular tolerance. A real surface can have out-of-plane error or form variation; an axial requirement can use a different zone shape and derived-feature construction. The arithmetic supplies no acceptance result for those cases.

## Modifiers and software scope

Preserve a modifier's exact placement and meaning. For example, PC-DMIS 2023.1 documents MMC/LMC options on specified controlled feature types, a projected-zone option for an Auto cylinder and a tangent-plane option for a plane with surface data. Those are distinct evaluations, not settings to select interchangeably to obtain a smaller result. [Hexagon allowed modifiers](https://docs.hexagonmi.com/pcdmis/2023.1/en/helpcenter/mergedprojects/core/geometric_tolerances/Perpendicularity.htm)

If the requirement uses a material condition, establish the relevant size characteristic before calculating any additional tolerance. A datum-reference modifier concerns reference establishment and must not silently become a numerical addition to the controlled feature's tolerance. The cited PC-DMIS page explicitly says that version 2020 R2 and later does not allow material modifiers on perpendicularity datum references. Resolve an unsupported or incomplete indication under the governing standard before choosing an inspection workaround.

## Proposed handoff record

Preserve the drawing revision, governing edition, feature identity and extent, complete indication, datum realization, sampling strategy, feature construction and software release/settings. Report the measured linear value with its units and the applicable uncertainty/decision rule. Retain raw data where needed to investigate differing results.

These are practical record fields rather than a normative report template. More displayed decimal places do not compensate for an unsuitable datum setup, incomplete coverage or the wrong feature construction.

## Source scope

The standard records identify editions and reaffirmation/confirmation years. The Hexagon implementation example is explicitly PC-DMIS 2023.1. KEYENCE's live tutorials have no verified technical revision date here. The segment calculation and proposed handoff fields are original explanatory material.

[Dimensions, tolerances and specifications](index.md)