Reference & Charts
GD&T Symbol Library
The 14 geometric characteristics of GD&T (ASME Y14.5) — form, orientation, location, profile, and runout — with each symbol, what it controls, and whether it needs a datum reference. Plus how to read a feature control frame.
Last verified: 2026-07-09
Geometric Dimensioning & Tolerancing (GD&T) controls the geometry of features — form, orientation, location, and runout — beyond what plus/minus dimensions can. Each control is called out in a feature control frame. The 14 characteristics below fall into five groups; form controls never use a datum, while orientation, location, and runout controls always reference one or more datums.
| Symbol | Characteristic | Group | Datum needed? | Controls |
|---|---|---|---|---|
| ⏤ | Straightness | Form | No | How straight a line element or axis is |
| ⏥ | Flatness | Form | No | How flat a surface is (all points in one plane) |
| ○ | Circularity | Form | No | Roundness of a single cross-section |
| ⌭ | Cylindricity | Form | No | Roundness + straightness of a whole cylinder |
| ⌒ | Profile of a line | Profile | Sometimes | Line-element deviation from a true profile |
| ⌓ | Profile of a surface | Profile | Sometimes | Whole-surface deviation from a true profile |
| ∠ | Angularity | Orientation | Yes | A feature at a specified angle to a datum |
| ⊥ | Perpendicularity | Orientation | Yes | 90° to a datum |
| ∥ | Parallelism | Orientation | Yes | Parallel to a datum |
| ⌖ | Position | Location | Yes | Location of a feature (e.g. a hole) from datums |
| ◎ | Concentricity | Location | Yes | Median points centered on a datum axis (rarely used) |
| ⌯ | Symmetry | Location | Yes | Median points symmetric about a datum plane (rarely used) |
| ↗ | Circular runout | Runout | Yes | Wobble of each cross-section about a datum axis |
| ⌰ | Total runout | Runout | Yes | Wobble of the whole surface about a datum axis |
Reading a feature control frame
A feature control frame reads left to right: the geometric characteristic, then the tolerance zone (with any diameter symbol ⌀ and material condition modifier), then the datum references in order of precedence.
- Example: ⌖ | ⌀0.25 Ⓜ | A | B | C — position, within a 0.25 mm diameter zone at maximum material condition, relative to datums A, B, C.
- Ⓜ (MMC) — the tolerance applies at the feature's maximum material condition; extra tolerance is gained as the feature departs from MMC (bonus tolerance).
- Ⓛ (LMC) — the tolerance applies at least material condition.
- No modifier — regardless of feature size (RFS), the default: the stated tolerance applies at every size.
Frequently asked
- Which GD&T controls need a datum?
- Form controls (straightness, flatness, circularity, cylindricity) never reference a datum — they control a feature against itself. Orientation, location, and runout always reference one or more datums. Profile can be used with or without datums.
- What is a datum?
- A datum is a theoretically exact reference (a plane, axis, or point) established from a physical datum feature on the part. Tolerances that need a reference are measured relative to datums, called out in precedence order (primary, secondary, tertiary).
- Position vs concentricity — which should I use?
- Position, almost always. Concentricity (and symmetry) are hard to inspect and were removed in ASME Y14.5-2018; position or profile achieves the same intent more practically.
Sources
- ASME Y14.5 — Dimensioning and Tolerancing (geometric characteristics & feature control frames)