Design Nomenclature / Systems & Spec
Basic dimension
Also called BSC, theoretically exact dimension, TED, boxed dimension
In GD&T, an exact theoretical value — boxed on the drawing — that locates or sizes a feature and carries no tolerance of its own. The tolerance lives in the feature control frame that refers to it, measured from the datums. ISO calls the same thing a theoretically exact dimension.
In practice
The box is the whole point and is the first thing lost in a redraw. An unboxed number silently inherits the title-block general tolerance instead.
Not to be confused with
- Nominal dimensionThe round-number size a feature is called by — 2×4 lumber, half-inch pipe, an M6 bolt. It names a class of part and is frequently not the measured size of anything in it. What governs manufacture is the specified size with its limits, which the nominal only points at.
- ToleranceThe total permissible variation on a dimension — the distance between its upper and lower limits. Bilateral tolerance splits the band either side of the stated size (10 ±0.1); unilateral permits deviation one way only (10 +0.2 / −0). The band is what gets manufactured to; the stated size is a convenience.