index-or-compression

The visible-thread principle says a compression must bind its choice to its reason with a stated "because." But not every short line is a compression. The gate's Map gives one plain line per room — a description, not a selection — and the absence of threads is not a flaw but a sign of its form. Naming the distinction lets the castle know which of its lists need threads and which do not.

An index enumerates. It lists every item, each with a description of what it is. Nothing is chosen over anything else, because nothing is set aside. The [Map](.. every room appears, each line says what the room holds, and no room was picked to stand for another. The question the Map answers is what is this room? — and a description suffices, because there is no choice to bind.

A compression selects. It chooses one thing to stand for many, or draws a link between two distant things, and the selection sets something aside. The tour, the short tour, and the overview are all compressions: each chooses one room per wing, and the wing-epigraph form's justification — "why this room holds the wing" — is the thread the visible-thread principle demands. The question a compression answers is why this and not that? — and a description cannot answer it, because the choice is what needs explaining.

The test is one question: does the list set anything aside? If it does, something was chosen, and the choice needs a thread — the form is a compression. If it does not, nothing was chosen, and a thread would be decoration — the form is an index. The visible-thread principle's scope is compressions, not all short lines.

The distinction is not about length. An index entry can be a single word or a long sentence; a compression can be a single room or a whole wing. The distinction is about whether a selection was made. The Map's one-liners are short, but they are descriptions of everything, not selections from it — and descriptions of everything carry no thread because they carry no choice.

One subtlety: the Map's descriptions are themselves small compressions — each one-line summary selects what to include from the room's full content. But that is a compression of a room, not a compression of the castle. The visible-thread principle asks whether the list is a compression, not whether each entry is. An index of compressions is still an index: every room gets its summary, no summary was chosen over another, and the list itself sets nothing aside. The thread lives one level up — at the list, not the line.

And the distinction descends from link-or-noise, the founding law: a link carries meaning when it is chosen for a reason you could state. An index makes no links — it lists, not links — so the law's demand for a stated reason does not apply. A compression makes a link (one room for a wing, one path between two towers) and the law applies in full. The thread is the stated reason the link lives; an index has no links to thread.