# Parallelism

Datum-based orientation tolerances for surfaces, axes, and median features, with practical interpretation and inspection limits.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, gd-and-t, metrology

Parallelism constrains orientation to a datum with a linear zone. Datum separation is a separate requirement. [GD&T Basics, parallelism](https://www.gdandtbasics.com/parallelism/)

In the ideal face example, slope 0.0004 over a 100 mm horizontal span gives **0.040 mm parallelism** and zero flatness. Translating the entire plane 1 mm upward leaves its 0.040 mm datum-normal range unchanged. This example uses a fixed ideal datum.

## Read the complete requirement

Start with the drawing or model's governing standard, edition, units, feature-control-frame attachment, datum references, zone-shape symbol, and modifiers. **ASME Y14.5-2018 (R2024)** is the publication identified by ASME; **ISO 1101:2017**, edition 4, is identified by ISO as confirmed in 2022. These public catalog records establish editions and scope; they do not make the systems' defaults interchangeable. Reaffirmation and confirmation dates are distinct from technical publication dates. [ASME Y14.5](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing/2018), [ISO 1101:2017](https://www.iso.org/standard/66777.html)

Identify both the controlled feature and the datum feature. A datum is an ideal reference, such as a plane or line; the actual datum feature is the imperfect physical geometry used to establish it. A manufacturing setup's machine coordinates are not automatically the drawing's datum reference. KEYENCE's datum introduction makes this ideal-reference distinction explicit. [KEYENCE, understanding datums](https://www.keyence.com/ss/products/measure-sys/gd-and-t/datum/)

## Surface, axis, and median-feature zones

| Application | Zone or evaluation | Interpretation check |
| --- | --- | --- |
| Ordinary surface parallelism | Two planes separated by the tolerance and oriented to the datum | Evaluate the surface, not only its fitted plane's angle. |
| Axis with a diameter zone symbol | A cylindrical zone with its axis oriented to the datum reference | The numerical value is the zone diameter, not a radial allowance. |
| Axis with a planar zone | Parallel planes controlling deviation in the indicated direction | Establish the zone orientation; this is not interchangeable with a cylindrical zone. |
| Width or midplane application | A planar zone applied to the specified derived feature | Measure enough of both opposing surfaces to construct the required feature. |

GD&T Basics illustrates the usual surface, cylindrical-axis, and midplane zones. Hexagon's **PC-DMIS 2023.2** documentation additionally supports diametric or planar zones for axial features and requires the planar zone's orientation to be specified. These software capabilities explain why a shorthand instruction such as “check the hole parallel” is incomplete. [GD&T Basics](https://www.gdandtbasics.com/parallelism/), [Hexagon, parallelism](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/Parallelism.htm)

## What parallelism does and does not constrain

An ordinary surface zone limits both form and orientation. Flatness permits independent zone orientation: a flat inclined face can fail parallelism. A tighter flatness callout can refine form separately. Do not extend this relationship uncritically to derived-axis or tangent-plane requirements. [GD&T Basics, surface parallelism and flatness](https://www.gdandtbasics.com/parallelism/)

Size and location remain separate requirements. GD&T Basics' **February 14, 2021** datum-reference-frame example explains that orientation zones retain translational freedom and require additional dimensions to locate the holes. Its example uses two mutually perpendicular datum planes to constrain the direction of a hole axis: parallelism to only one plane leaves an in-plane direction free. Datum references must be interpreted in their stated order. Position may be appropriate when both location and orientation need control, but replacing a callout requires design authority. [GD&T Basics, feature-of-size parallelism](https://www.gdandtbasics.com/parallelism-feature-of-size/)

Material-condition indications also require feature-specific interpretation. Do not apply a hole's size-dependent allowance to an ordinary surface requirement or infer extra tolerance from a caliper diameter without checking the applicable size definition and governing standard. The same teaching source distinguishes material modifiers on features of size from surface applications. [GD&T Basics, orientation rules](https://www.gdandtbasics.com/parallelism-feature-of-size/)

## Inspection and association

For a suitable rigid part with a planar datum feature, a surface plate and indicator can provide a practical surface check. With the datum properly established, scan the controlled region and take the highest minus lowest reading in the datum-normal direction. Cover the area rather than assuming one traverse represents the complete face. KEYENCE notes that stylus force can deform non-rigid parts and that a reference face which cannot be seated properly makes this setup difficult. Its page is undated; its illustrative point counts are not universal sampling requirements. [KEYENCE, measuring parallelism](https://www.keyence.com/ss/products/measure-sys/gd-and-t/orientation-tolerance/parallelism.jsp)

For coordinate measurement, distinguish datum construction from controlled-feature construction. PC-DMIS 2023.2's parallelism help, dated **August 30, 2023**, requires correct nominal geometry and optimizes the zone subject to datum constraints. Its sparse-sampling illustration produces a measured value below the actual value. Accordingly, retain acquisition coverage on both the controlled and datum features; a dense target scan cannot compensate for a poorly established reference. [Hexagon, parallelism evaluation](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/Parallelism.htm)

Hexagon's companion datum documentation, also dated August 30, 2023, distinguishes DEFAULT and LSQ datum math. For its ASME implementation, DEFAULT applies void filtration to planar surfaces and constrained least-squares fitting; LSQ uses an ordinary unfiltered least-squares fit. The ISO implementation offers different DEFAULT constructions. These are edition-specific software descriptions, not permission to choose the most favorable result. Record the selected standard and datum math, and justify the measurement strategy against the required datum simulation and uncertainty. [Hexagon, how PC-DMIS solves datums](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/How_PC-DMIS_Solves_Datums.htm)

An axis is likewise not simply whatever cylinder fit was used for a size report. Hexagon's feature-derivation documentation distinguishes ASME fitted/extrapolated axes from its ISO treatment, which can construct an imperfect axis from sectional centers under stated conditions. Acquisition layout, feature math, and evaluated length therefore belong in an axis-parallelism report. Avoid assuming that the straightness of one fitted axis proves the form of the actual bore. [Hexagon, deriving the toleranced feature](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/Deriving_the_Toleranced_Feature.htm)

## Original calculation: form, orientation, and location

Assume an ideal datum plane `z = 0` and a perfectly planar controlled face:

`z = 20.000 + 0.0004 x`, over `0 ≤ x ≤ 100 mm`.

Its datum-normal height range is `0.0004 × 100 = 0.040 mm`. Thus its surface parallelism value to this ideal datum is **0.040 mm**, while its mathematical flatness is **zero**. A hypothetical 0.030 mm parallelism requirement is exceeded by 0.010 mm. The corresponding inclination is `arctan(0.0004) ≈ 0.0229°`; that angle explains this ideal ramp but does not replace the distance-based tolerance.

Raise the entire face by 1 mm: `z = 21.000 + 0.0004 x`. The parallelism value remains **0.040 mm** because both extremes increased equally. Whether the new separation is acceptable depends on a separate size or location requirement. This reconciled example assumes perfect surfaces and datum, complete coverage, and no measurement error; real acceptance also requires the applicable uncertainty and decision rule.

## Information to retain

Retain the controlled feature and extent, datum sequence, standard edition, zone shape and direction, modifiers, size results where relevant, acquisition coverage, feature/datum association, software version, support condition, and decision rule. Document restraint for flexible parts rather than silently clamping them into a different shape. Diagnose differing results by comparing these inputs before changing the tolerance or manufacturing process.

## Related reference

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