The theoretically exact location of a feature, from which positional tolerance is measured.
True position is a diameter-based tolerance zone centred on the basic (boxed) dimension. A position callout of ⌀0.5 means the feature's axis must lie within a 0.5 mm diameter cylinder centred on true position — not within 0.5 mm in each direction, which would be a square zone and a much larger tolerance.
The round zone is deliberate: it gives equal tolerance in every direction, which is what a fastener needs.
Reading a ⌀ position tolerance as a ± tolerance. ⌀0.5 is not ±0.25 — the round zone is 27% larger in area than the equivalent square, deliberately.
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