Dimensioning notation · Dimension type

Dimension Not to Scale — The Underline Convention

A straight thick underline beneath a dimension value, stating the drawn geometry is not at the stated scale — the printed value governs, the picture does not.

Definition

Agreement should exist between the drawing's pictorial proportions and its stated values. When a change breaks that agreement — a dimension re-valued without the geometry being redrawn — the remedy is a straight, thick, solid underline beneath the value: 215 is deliberate at its stated size, even though the picture no longer matches it.

The thick straight underline beneath the value is the not-to-scale convention: the arm was re-valued without redrawing, so the geometry no longer matches the stated dimension. The value governs; measure nothing off the print. The underline changes no tolerance.

Why the convention survives CAD

Scaled dimensions off a print were once routine — and dangerous; the underline is the drawing's formal warning that scaling will mislead. Even in model-driven workflows it marks the exact revision where geometry and values diverged, which is often the clue that a redraw or a changed part number is pending.

What the underline does not do

It changes no tolerance — the underlined value carries exactly the tolerance it would carry ununderlined
It is not a flag for inspection — the value is inspected normally
It applies to that dimension only; the rest of the view stays to scale
A whole view drawn distorted carries a NOT TO SCALE note rather than underlining every value

Related honesty conventions

Long features broken with a zigzag break line, foreshortened dimensions shown shorter than true, and features too small to draw at scale all share the underline's logic: the value is authoritative, the geometry is a sketch of it. None of these conventions change a single tolerance.