What "Exact Outline" Really Means

The rule, word for word

The keyholes instructions close with a promise about the answer: the opening is always the exact shape of the appropriate external outline of the object. Three words in that sentence carry the whole section — exact, external, and outline — and each one throws out a different kind of tempting wrong answer.

"Exact" means zero clearance

There is no tolerance here. The correct opening is not a comfortable fit around the object; it is the object's boundary, edge for edge.

That cuts both ways. An opening slightly too small blocks the object — the material simply will not go through. An opening slightly too large would let the object drop through easily, but it is no longer the exact shape of any outline, so it is not the answer either. You are not hunting for an opening the object fits through. You are hunting for the one whose boundary is the object's boundary.

Exact outline — passes

Squeezed 15% — too tight

None of this is decided by a hairline. When the generator offers you a shrunken copy of a true outline, it shrinks it to somewhere between 68 and 82 percent of full size — a fifth to a third too small, drawn at the one shared scale every figure in an item uses. That is a gap you can see. A later lesson catalogs the specific ways wrong openings miss.

"External" means the boundary, not the surface

An outline is a boundary and nothing else. Whatever happens inside it — a raised pad, a step between two levels, a boss standing proud of a face — leaves no mark on the aperture, because none of it moves the boundary.

Seen from above, that cross boss is the most conspicuous thing on the disc. Here is the opening the same view produces.

TOP VIEW

The boss is gone. It sits entirely within the disc's circle, so it shifts no part of the edge. The notch survives, because it is cut clean through the rim and takes a bite out of the boundary itself. The question to ask about any feature is never "is it big?" but "does it reach the edge?"

An opening never contains an island

Now the rule that wins you the most points in this unit, and the one most students get backwards.

An aperture is a hole cut in a plate. A ring floating inside it would be a disc of plate attached to nothing — it would simply fall out. So no matter what the object looks like, the correct opening never encloses a ring. If an option has a closed loop inside its boundary, it is wrong. You can eliminate it without a moment's geometry.

That rule is worth so much because the trap is genuinely tempting. Objects really do have bores driven through them, and looking straight down a bore you really do see daylight all the way through.

Traced with the bore — always wrong

External outline — the opening

The left panel is what a student draws when they trace what they see: the wall's boundary with the bore left open inside it. It is an honest drawing of the shadow, and it is never the answer. The right panel is the external outline — the same shape with the hole filled — and that is the opening. Filling a hole only makes an opening roomier, so the object still passes through it comfortably.

The bore has not been ignored, either. It matters everywhere except here: look at that wall from the side instead and the bore vanishes completely, because solid material stands in front of it and behind it. A pocket, a blind hole, or a groove that stops partway never shows in any outline at all, for the same reason — material behind it closes the gap.

Expect at most one option like this per item, and expect it to look plausible. It is drawn from a real view of a real object; only the rule disqualifies it.

Exactly one opening is right

Five openings, one answer — never two, never none. One of them is the exact outline of one of the object's sides. Of the other four, the traced-with-holes option is wrong by the outline rule alone, and every remaining one has been tested against the object and shown to reject it in every orientation it could be turned to before insertion. The five are also kept visibly different from one another, so you will never find two options that are the same opening drawn at two rotations. If two candidates both look right, you have misread one of them.

A

B

C ✓

D

encloses a ring

E

The correct opening is the object's front outline.

Here both rules earn their keep. Option D traces the object faithfully and encloses a ring, so it goes out on sight — no measuring required. The answer is C, and notice what it looks like: a plain boundary with nothing inside it, which is what every correct opening looks like.

Put the rule to work in the keyholes practice generator.

The takeaway

Exact, external, outline. Exact rules out any opening tighter or looser than the object's own boundary. External rules out every interior pad, step and boss that does not reach the edge. And outline means a boundary with nothing inside it: an option carrying a ring is always wrong, however faithfully it traces the object.