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DATUM / MODIFIER GUIDE

BASIC DIMENSION

A basic dimension states a theoretically exact location, angle or profile. It has no separate plus/minus tolerance: the associated geometric specification defines permitted variation around the exact geometry.

Datum required
Depends on the associated geometric control
MMC / LMC applicable
No — exact dimension
Control type
Theoretically exact geometry
What it controls
Nominal zone location or orientation
Typical features
Hole centres, angles and profiles

INTERACTIVE APPLICATION

Basic Dimension: drawing and interpretation

Boxed basic locations 20 from datum B and 10 from datum A, with position tolerance diameter 0.22010AB⌀0.2AB
DRAWING CALLOUT

Read the annotation

Boxed 20 and 10 locate the theoretically exact hole centre from datums B and A. The enlarged view shows the actual axis inside the circular ⌀0.2 position zone; the value specifies the full diameter.

Key points

  • Basic dimensions locate and orient the tolerance zone; the actual feature may depart within its geometric tolerance.
01 / PURPOSE

Why use this annotation?

Separate the nominal geometry from the allowed error so a geometric tolerance can describe the functional requirement directly.

02 / APPLICATION

Where is it applied?

Hole patterns, datum-target locations, angularity angles and true profiles.

03 / DRAWING

How to apply and read the annotation

Boxed basic locations 20 from datum B and 10 from datum A, with position tolerance diameter 0.22010AB⌀0.2AB
Original schematic. Example values use millimetres; a production drawing needs its complete requirements.

Steps to add it to a drawing

  1. Define the required exact geometry relative to the reference system.
  2. Box the numeric dimension using the drawing convention.
  3. Add the geometric requirement that controls actual variation.
  4. Keep independent feature-size limits, such as a hole diameter, separate.

How to read it

Boxed 20 locates the centre from datum B; boxed 10 locates it from datum A. The position frame limits the hole axis to a ⌀0.2 zone centred at that exact location. Hole size and any additional production requirements remain separate.

Common mistake

A boxed number by itself does not define acceptable deviation. Check the associated control, profile definition or drawing-wide requirement.

04 / VALUES

Example values and identifiers

These explain the drawing notation. They are examples, not recommended default tolerances.

Basic dimension 20 enclosed in a rectangular box20

Theoretically exact distance.

Basic angle 30 degrees enclosed in a rectangular box30°

Theoretically exact orientation.

Position feature control frame with diameter 0.2 and datums A and B⌀0.2AB

Allowed geometric variation; not a basic dimension.

05 / INSPECTION

Checking the requirement in practice

Establish the datum system, construct the theoretically exact geometry, then evaluate measured features using their geometric tolerance. Do not reject a feature simply because a coordinate is not exactly equal to the boxed value.

Choose sampling, fixtures and acceptance rules that suit the feature and the drawing’s cited standard.

Practical inspection of basic dimensionAEvaluate the stated requirement

ASME Y14.5-2018 (R2024)

ASME Y14.5 commonly boxes basic dimensions and controls variation with a related geometric tolerance.

ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.

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