Mechanical Variation & GD&T/GPS Terms

A

Actual local size

The measured size of a feature at a specific cross-section or location, used when evaluating size and form against GD&T/GPS requirements.

Actual mating envelope

The smallest (for external features) or largest (for internal features) perfect geometry that can be placed over or within a real feature so that it just makes contact, used for RFS and MMC concepts.

All around symbol

A drawing symbol that indicates a requirement (often a profile tolerance) applies all the way around a part or feature, not just to the portion directly adjacent to the feature control frame. It can be envisioned as placing a single straight piece of tape around a part such that the tape begins and ends in the same place.

All over symbol

A drawing symbol that indicates a requirement applies to all surfaces of the part, whether those surfaces are on the outside or inside of the part.

Arc length symbol (⌒)

A symbol that shows a dimension applies to the length along an arc rather than to a straight-line distance.

ASME GD&T

The language defined within ASME Y14.5 that uses rules, symbols, and practices for communicating allowable geometric variation on engineering drawings and 3D models.

Assembly sequence

The defined order in which components are assembled; different sequences can change how variation accumulates and how gaps, flush conditions, and alignments behave.

Angularity (∠)

An orientation control with the symbol ∠ that limits how a surface, axis, or center plane is tilted away from a specified basic angle (typically excluding 0°, 90°, or 180° angles) relative to a datum reference frame.

B

Balance cost and quality

An engineering and manufacturing goal of choosing tolerances that are optimized to protect functional requirements and quality with minimum corresponding machining, inspection, and supplier costs.

Basic dimension (Theoretically Exact Dimension)

A theoretically exact value, often shown boxed on a drawing, that defines the ideal geometry of a feature; used when a geometric tolerance is associated to a feature.

Bonus tolerance

Additional usable geometric tolerance that becomes available when a feature departs from a specified material condition, rewarding processes that produce parts away from the protected size limit.

Build variation

All sources of variation that show up during assembly, including part tolerances, fixtures, tooling, operator variability, and assembly methods, which together influence product performance and yield.

C

Capability index (Cpk)

A statistical metric that compares the spread and centering of a process to its specification limits, indicating how consistently the process can produce parts within tolerance.

CETOL 6σ

Sigmetrix 3D tolerance analysis software that models how dimensional and assembly variation stack up across an entire product, helping teams predict performance and make informed decisions about tolerances before releasing designs.

Circularity / roundness (◯)

Circularity (roundness in ISO GPS), with the symbol ◯, is a form control that limits how much any cross-section of a cylindrical or conical surface can deviate from a perfect circle, independent of any datums.

Circular runout (➚)

Circular runout, with the symbol ➚ in the feature control frame, is a runout control that limits the variation of a surface element as the part is rotated 360 degrees about a datum axis, evaluated at individual circular cross-sections but applicable to an entire feature.

Coaxiality (Ⓞ)

Coaxiality has two meanings: it describes a condition where multiple cylindrical features share the same axis, which in modern ASME practice is usually controlled with a position tolerance referencing a datum axis, and it is also a location control in ISO GPS that limits how far the axis of a feature may deviate from a datum axis, equivalent to position in ASME.

Concentricity (◎, ACS in ISO)

Concentricity, with the symbol ◎, is a location control in ASME that limits how far the median points of opposed point pairs of a feature of size may deviate from a datum axis. In ISO, concentricity with the symbol ◎ and the note “ACS” adjacent to the tolerance indicator limits how far the centers of each circular cross section may vary from the local datum point. It is different from both ASME concentricity and ISO coaxiality.

Conical taper symbol (ASME only)

A dimension specified as a ratio that indicates the value is a taper ratio expressing diameter change over length for a conical feature. The ratio value must always be relative to one unit, such as 1.5:1 or 2:1.

Continuous feature (CF) (ASME only)

The continuous feature symbol (CF) identifies two or more separate features that function as a single feature for size and geometric controls. When CF is applied, the grouped features share one common tolerance zone and are evaluated as if they form one continuous feature of size, even if there are intentional gaps or steps between them.

Control plan

A structured document that lists key characteristics, measurement methods, sampling plans, and reaction plans used to monitor manufacturing and assembly processes.

Controlled radius symbol (CR) (ASME only)

A radius symbol variant, shown as CR, used when a contour must form a smooth, fair curve with no sudden reversals, flats, or sharp changes in curvature.

Countersink symbol (⌵)

A symbol that indicates a conical enlargement at the opening of a hole, typically sized and angled to match a flat-head fastener so it can sit flush with or below the surface.

Counterbore symbol (⌴)

A symbol that indicates a flat-bottomed cylindrical enlargement at the opening of a hole, used to seat the head of a fastener or create clearance for tooling.

Cylindricity (⌭)

Cylindricity, with the symbol ⌭, is a form control that limits how much a cylindrical surface can deviate from a perfect cylinder.

D

Datum feature

A physical feature on a part, such as a surface or hole, that is used to establish a datum reference frame for measurement and assembly.

Datum reference frame (DRF, ASME) (Datum System, ISO)

A coordinate system built from one or more datum features in a specific order of precedence used for defining and measuring the location and orientation of features on a part or assembly.

Datum target symbol

A target symbol with a letter and number (for example A1) that identifies a specific point, line, or area on a part used to establish a datum when the full surface is not suitable by itself.

Depth symbol (↧)

A symbol placed with a value to indicate the depth of a hole, pocket, or recess from the outer surface of a part.

Design for manufacturability (DFM)

An approach to product design that considers process capabilities, tooling, and variation prior to production release or prototyping so that parts and assemblies can be built reliably at the desired cost and quality levels.

Dimension origin symbol

A small circle replacing an arrow at one end of a dimension line that indicates the origin or starting point from which a linear dimension is measured.

Dimensional inspection plan

A documented plan that defines which dimensions and geometric tolerances will be checked, how they will be measured, and at what frequency.

Dimensional management

The practice of planning, analyzing, and controlling dimensional variation across the product lifecycle so that assemblies meet their functional requirements consistently.

Digital thread

The connected flow of product information from requirements and design through manufacturing, inspection, and service so that each team works from consistent, up-to-date data.

Diameter symbol (⌀)

A symbol ⌀ used to indicate that the associated dimension applies across the full diameter of a cylindrical or circular feature or area.

E

Envelope principle (ASME Rule 1)

A rule that states the form of a regular feature of size is controlled by its limits of size; the entire surface must fit inside (or around) a perfect envelope defined by that size.

Envelope requirement symbol (E in a circle) (ISO only)

A symbol often represented as E in a circle that is used to invoke the Envelope Principle explained above. This requirement is not a default in ISO GPS.

EZtol

Sigmetrix 1D tolerance stack-up analysis tool that runs directly on top of popular CAD systems, helping teams quickly evaluate and adjust linear chains of dimensions in assemblies without leaving their CAD environment.

F

Feature control frame

The rectangular GD&T frame that states which geometric characteristic symbol is specified, the tolerance value and modifiers, and any datum reference frame used as a basis for that control.

Feature of size (FOS)

A feature such as a hole, pin, slot, or tab that can be controlled using maximum or least material concepts.

Flatness (⏥)

Flatness, with the symbol ⏥, is a form control that limits how much a surface may deviate from a perfect plane, independent of any datum reference frame.

Free-state condition

The natural shape of a part when it is not being held by fixtures or assembly forces, especially important for flexible or thin-walled components.

Free state symbol Ⓕ

A modifier (Ⓕ) that indicates a dimension and its tolerance are to be assessed with the part in a Free-state condition.

Functional gage

A gage designed to verify that a feature or part satisfies its GD&T/GPS requirements by simulating how it assembles or functions, rather than measuring each dimension or tolerance separately.

Functional requirement

The specific behavior or performance a part or assembly must achieve in its operating environment, such as fit, alignment, motion, or sealing.

G

GD&T (Geometric Dimensioning and Tolerancing)

A symbolic language defined by ASME Y14.5 for specifying the allowable variation of part geometry in a way that relates directly to functional requirements and inspection.

GD&T Advisor

Sigmetrix software that guides users in applying GD&T/GPS inside supported CAD systems, helping ensure that annotations are complete, consistent, and aligned with the governing standards.

GD&T/GPS

A shorthand way to refer to both the ASME GD&T and ISO GPS standards together, acknowledging that organizations may use either set of rules to define and verify product geometry.

Geometric tolerancing

The practice of controlling the size, shape, orientation, and location of features using geometric tolerances rather than relying only on plus/minus size limits.

H

High-low assembly strategy

An intentional pairing approach where larger features are matched with smaller mating features or vice versa to influence fit, contact patterns, or performance.

Hole pattern

A group of holes arranged in a pattern whose relative locations and orientations are often controlled with GD&T/GPS controls such as position or perpendicularity.

I

Independency symbol (ⓛ) (ASME only)

A symbol (ⓛ) that indicates the requirement for perfect form at maximum or least material condition is removed, permitting size and form to be controlled independently.

ISO GPS (Geometrical Product Specification)

A family of ISO standards that define how to specify and verify the geometry of parts and assemblies, similar in purpose to ASME GD&T but with different terminology and rules and additional complexity.

K

Key characteristic (KC)

A dimension or feature whose variation has a strong impact on functional requirements, safety, performance, or customer satisfaction and therefore receives additional control and monitoring.

L

Least material condition (LMC)

The condition of a feature of size where it contains the least amount of material, such as the largest hole or smallest shaft, often used when minimum wall thickness or minimum clearance is crucial.

Least material condition symbol (Ⓛ)

A modifier (Ⓛ) indicating that a geometric tolerance has no additional tolerance at the feature’s least material condition, but additional tolerance is available as the feature departs from that condition.

Limit dimensioning

A directly toleranced dimensioning method that specifies only upper and lower numeric limits instead of a nominal value with a separate tolerance. Common in drawings that do not use GD&T/GPS extensively.

M

Maximum material condition (MMC) (ISO MMS)

The condition of a feature of size where it contains the most material, such as the smallest hole or largest shaft. Useful concept to ensure fit of mating parts.

Maximum material condition symbol (M in a circle)

A modifier shown as M in a circle, indicating that a geometric tolerance is applied at the feature’s MMC and that additional bonus tolerance is allowed as the feature departs from that condition.

Mechanical variation (also Dimensional variation)

The differences between nominal design geometry and the actual manufactured parts and assemblies caused by manufacturing processes, fixtures, measurement methods, and operating conditions.

Mechanical variation management (MVM) (also Dimensional Management)

A systematic approach to understanding, predicting, and controlling mechanical variation across the product lifecycle so that teams can meet functional requirements, reduce rework, and improve yield.

MBD-ready model

A CAD model that has been structured, organized, and annotated so that it can reliably serve as the source for model-based definition, including appropriate PMI and clean, consistent geometry.

Model-based definition (MBD)

The practice of using the 3D CAD model as the primary, authoritative source of design specifications by embedding dimensions in the model; GD&T/GPS, notes, and other product information may be specified on an accompanying drawing or in the model.

Model-based enterprise (MBE)

An organization that extends MBD beyond engineering so that downstream groups in manufacturing, quality, supply chain, and service consume and act on model-based information throughout the product lifecycle.

N

Number of places symbol (for example 4X)

A notation such as 2X, 3X, or 4X that indicates a single dimension applies to several identical features at once, with the number showing how many features are included.

O

Orientation tolerance

A family of geometric tolerances, including parallelism, perpendicularity, and angularity, that control how a feature is oriented relative to a datum reference frame without directly fixing its location.

P

Parallelism (//)

Parallelism, with the symbol //, is an orientation control that limits how parallel a surface or axis must be relative to a datum plane or datum axis, without directly constraining its location.

Parting line symbol (PL)

A symbol or note, often PL, that identifies where the separate halves of a mold or die meet; if it is not specified, the manufacturer typically selects the parting line location based on the shape of the part.

Perpendicularity (⊥)

Perpendicularity, with the symbol ⊥, is an orientation control that limits how close a surface or axis must be to 90 degrees relative to a datum plane or datum axis.

Planarity

The measure, qualitative or quantitative, of a surface’s semblance of a perfect geometric plane; this is typically limited using a flatness tolerance.

PMI (Product and Manufacturing Information)

Non-graphical information such as dimensions, GD&T/GPS, notes, and surface finish that is associated with 3D geometry in CAD models and used to communicate design specifications to downstream users.

Position tolerance (⌖)

Position, with the symbol ⌖, is a location control that defines how far the actual location of a feature of size may deviate from its theoretically exact location relative to a datum reference frame.

Process capability

A measure of how well a process can consistently produce an output within specified tolerances, often summarized with indices such as Cp and Cpk.

Process variation

The natural fluctuation that occurs in a manufacturing and assembly process, driven by machines, methods, materials, environment, and human factors.

Product and process knowledge

Information that organizations aim to capture and reuse about how a product is designed to function and how it is manufactured, including tolerances, key characteristics, and process capabilities; often used to guide improvement over time.

Profile tolerance

A geometric tolerance that controls how far the surface or outline of a feature can deviate from its ideal geometry in both orientation and location. Known as the only type of geometric tolerance capable of controlling complex, freeform shapes.

Profile of a line (⌒)

Profile of a line, with the symbol ⌒, is a profile control that limits the deviation of a cross-sectional curve from its ideal profile, evaluated in individual sections along a feature. Similar to the cross-section of a profile of a surface tolerance in the specified direction.

Profile of a surface (⌓)

Profile of a surface, with the symbol ⌓, is a profile control that limits how far an entire surface may deviate from its ideal 3D profile. Profile of a surface can control form, orientation, and sometimes location in a single tolerance.

Projected tolerance zone symbol (P in a circle)

A modifier represented as P in a circle that indicates the tolerance zone for a feature extends beyond the surface of the part by a specified distance, typically for threaded holes, studs, or pins that affect components some distance away.

R

Radius symbol (R)

A symbol R preceding a numeric value indicating that the dimension specifies the radius of an arc or rounded feature rather than its diameter.

Regardless of feature size (RFS)

The default condition, often implied, in which a geometric tolerance is applied independently of the actual size of the feature, without any bonus tolerance tied to maximum or least material condition.

Regardless of material boundary (RMB)

The default condition, often implied, in which a datum feature is referenced independently of the actual size and geometric variation of the datum feature. When RMB is specified, there is not intended to be any datum feature shift.

Repeatability

The degree to which repeated measurements under the same conditions yield the same result, often evaluated as part of a gage repeatability and reproducibility (R&R) study.

Residual variation

Remaining variation in a measurement after design changes, process improvements, or tolerance adjustments have been applied.

Restrained condition note

A note that indicates a flexible or deformable part is to be restrained in a specific way during inspection so that dimensions and tolerances are evaluated in a defined restrained state instead of a free state.

Runout tolerance

A family of controls, including circular runout and total runout, that limit how much a surface can vary as the part is rotated around a datum axis, capturing combined effects of form, orientation, and sometimes location.

Runout (circular runout, ↗)

See circular runout.

S

Scrap rate

The percentage of parts or assemblies that must be discarded because they do not meet design specifications or functional requirements.

Sensitivity analysis (tolerance analysis)

An evaluation that shows how sensitive a measurement or functional requirement is to a variation in each contributing dimension, helping teams identify which tolerances have the greatest impact.

Slope symbol (⌳)

A notation, often dimensioned as a ratio (such as 0.65:1), that indicates a surface is controlled as a slope value representing change in height over a given length for a flat taper-style feature.

Spherical diameter symbol

A symbol or notation that indicates a diameter dimension applies to a spherical feature, defining a 3D spherical tolerance zone rather than a cylindrical one.

Spherical radius symbol (SR)

A symbol SR indicating that a radius dimension applies to a spherical surface rather than a planar arc. A spherical radius symbol may be used to define a spherical tolerance zone between minimum and maximum radii.

Spotface symbol (SF)

A symbol or note, often SF within a 3-sided box, that indicates a shallow counterbore-style feature used to create a small, flat seating surface for a fastener head or washer on an otherwise rounded or angled surface without removing more material than necessary.

Square symbol (□)

A symbol □ that indicates a dimension applies to a square cross section or square feature. The value following the square symbol indicates the length of each side.

Stack-up path

The sequence of dimensions and features that connect one functional surface or feature to another within an assembly at a specific design point; used when setting up tolerance stack-up analyses.

Statistical process control (SPC)

A method of using control charts and statistical techniques to monitor process behavior, detect shifts, and react before output exceeds tolerance.

Statistical tolerance modifier

A drawing notation (elongated hexagon containing the letters ST) that indicates a tolerance is intended to be managed using statistical methods such as SPC so that the combined effect of multiple parts is controlled statistically instead of purely by worst-case limits. The use of a statistical tolerance modifier is typically accompanied by a hard limit dimension.

Statistical tolerance analysis

An analysis approach that predicts the distribution of a measurement by modeling the statistical behavior of related individual dimensions, often assuming process distributions rather than relying only on worst-case limits.

Straightness (ー)

Straightness, represented by a straight line symbol ー, is a geometric form control that limits how much a line element or centerline may deviate from a perfectly straight line, commonly used for shafts, pins, and surfaces.

Surface profile tolerance

A profile control applied to entire surfaces, limiting overall form, orientation, and sometimes location of the surface relative to a datum reference frame.

Supplier capability

The ability of a supplier to hold specified tolerances and deliver acceptable variation over time, typically measured with capability indices and defect rates.

Symmetry (⌯)

Symmetry, with the symbol ⌯, is a location control that limits how evenly a feature is distributed about a datum center plane. In many modern designs, position or profile controls are used instead because they are more likely to control functional requirements and are easier to measure and interpret.

T

Tangent plane symbol (Ⓣ)

A modifier that indicates the geometric tolerance applies to a theoretical tangent plane contacting the highest points of a surface, rather than the entire surface itself.

Time to market

The total time required to progress a design from its concept to a launched product.

Tolerance

The allowed amount of variation from nominal, typically specified in a design document, ensuring the part or assembly satisfies its geometric functional requirements.

Tolerance analysis

The process of evaluating how dimensional variation across parts, features, and processes affects a measurement or functional requirement in an assembly.

Tolerance stack-up (tolerance stackup)

A method for calculating how multiple dimensions and tolerances combine along a stack-up path to affect a functional gap, alignment, or other characteristic.

Total runout (two runout symbols connected at the bottom with a horizontal line)

A runout tolerance that controls variation of an entire surface as the part is rotated about a datum axis, capturing combined form, orientation, and location effects over the full length or area.

True position

The theoretically exact location of a feature controlled by a position tolerance.

True profile

The theoretically exact location and form of a feature controlled by a profile tolerance.

1D tolerance analysis

A tolerance stack-up study focused on a linear chain of dimensions, typically along a single axis or measurement path; often used early in design or for simpler portions of mechanisms.

3D tolerance analysis

A geometrically comprehensive tolerance analysis that uses 3D models to account for multiple directions, rotations, and interactions between features throughout an assembly.

Unequally disposed profile symbol (Ⓤ)

A modifier associated to a profile tolerance that indicates the tolerance zone is not centered around the ideal profile and allows an unequal or unilateral distribution of a profile tolerance having a constant offset from the true profile.

Unilateral tolerance

A tolerance scheme that allows variation in only one direction from the nominal value, such as +0.2 / −0, often used when one side of the tolerance band is more important for function.

V

Variation analysis

A general term for studying how dimensional and assembly variation flows through a design and impacts the performance, quality, and yield of the final product.

VariSight

Sigmetrix software that extends mechanical variation management beyond individual analyses, making variation insights more accessible across programs, teams, and the product lifecycle.

Virtual condition

A boundary generated by the combined effect of a feature’s size and its geometric tolerance, often representing the worst acceptable condition that must still assemble or function.

W

Worst-case tolerance analysis

A tolerance analysis method that assumes all contributing dimensions are at their most extreme limits in the worst possible combination, ensuring assembly function regardless of individual part variation. Worst-case tolerance analysis often leads to tighter-than-necessary tolerances.

Y

Yield

The proportion of units produced that meet all design specifications and can be used without rework; directly influenced by how tolerances and processes are chosen.