Dimensioning notation · Dimension type

Reference Dimension — Parenthetical, Information-Only Values

A dimension repeated for information only, shown in parentheses: it contributes nothing to production or inspection because other dimensions already govern its value.

Definition

(100) — the parentheses are the entire symbol. A reference dimension either repeats a dimension stated elsewhere on the drawing or is derivable by adding and subtracting other dimensions. It exists for the reader, not for the part: production and inspection must take the value from the governing dimensions, never from the parenthetical.

The parenthesized (100) is a reference dimension: the three toleranced chain segments (25, 40, 35) already govern the overall length, so the (100) only informs. It is never inspected, and appending a tolerance to it would re-govern what is already governed.

Why drawings need them

Chain and overall dimensions are the classic case. Three segments dimensioned 25, 40 and 35 already fix the overall length — dimensioning it again as a normal 100 would add a second, independent tolerance over the same distance, and the two authorities could disagree: all three segments in tolerance yet the total out of tolerance, or the reverse. Written as (100), the overall length informs without governing, and the chain dimensions stay the single source of truth.

Never add a tolerance to one

A reference dimension with a tolerance appended is a drawing error — it re-governs what is already governed
Inspectors do not accept or reject parts against a reference value
If the value actually needs to govern, remove the parentheses and delete whichever dimension made it redundant
On dual-dimensioned drawings the alternate-unit value in brackets is likewise non-governing

Reference vs. basic, once more

Both appear without tolerances, and both get misread as important. Basic (boxed) is theoretically exact and its tolerance lives in the geometric controls. Reference (parenthesized) is derived information with no tolerance anywhere. One anchors the geometry; the other only annotates it.

Measurement

Inspection reports — reference values are excluded from ballooned characteristic lists; raising a characteristic on a parenthetical dimension is a planning error.

Layout checks — a layout may record the overall length for information, but acceptance criteria cite the chain dimensions that govern it.