Ambiguous Apertures

Ambiguous Apertures are openings in the fabric of the Tower whose dimensions, borders, and in some readings existence cannot be fixed with sufficient confidence to be entered under a single heading. Unlike Standard Apertures, which admit of measurement and are listed in the Register of Fixed Openings, an Ambiguous Aperture resists the surveyor's art: it may widen while being measured, narrow while being described, or prove on re-inspection to have been a shadow. The Bureau of Ambiguous Transit is the sole body empowered to issue manifests for passage through them, and has done so since its founding instrument, Third Concordance, Annex 7, was sealed in the year recorded on the lintel of Floor 404, which is itself disputed.

Definition and classification

The working definition is that of the Glossary of Contested Openings, recension four: an Ambiguous Aperture is any transit between two floors which cannot be assigned a width, a height, and a direction simultaneously. The Warden of Apertures maintains, and has maintained in three successive memorials, that the variability belongs to the aperture and not to the instrument — a position resisted by the Department of Ambiguous Metrics, whose surveyors hold that the fault lies in the measurement. The dispute has never been settled and is now older than the flights involved.

Passage falls into three rough grades, though the Committee on Aperture Grades has twice declined to fix their number as final:

• Grade I — apertures whose width varies but whose position is stable. These are safe to cross provided the traveller does not look at the edges.
• Grade II — apertures stable in width but mobile in position. Crossable by rope.
• Grade III — apertures that vary in both, and occasionally in a third respect the Committee declines to name in the public register.

Measurement and instrumentation

Survey work is conducted with Ambiguous Calipers, which yield readings in Standard Drift Units. A reading of one drift unit or fewer is considered compliant; readings above four are referred to the Vault of Ambiguous Records for a ruling, and are frequently filed as Gamma-Class when the aperture is suspected of existing only partly. Cadastral surveyors of the Bureau of Ambiguous Real Estate attend all Grade II crossings to issue sealed affidavits confirming that the space traversed was real for the duration of the crossing and not thereafter.

The Protocols for the Calibration of Ambiguous Calipers require calibration against a known opening before each expedition. On Floor 1131, in the winter of the year the boilers failed, a survey party calibrated against a door that was later found to be a painted panel, and spent eleven days measuring a corridor that did not shorten. Their report is preserved and is still cited as the standard example of what the Protocols do not prevent.

Transit procedure

A manifest must be obtained in advance. It states the traveller's name, the intended floor, and a description of the aperture sufficient to distinguish it from any other aperture the Bureau has on file, which on crowded floors may require reference to a numbered annex. Manifests are not transferable and are void if the aperture is found to have moved more than six feet sideways between issue and use. A traveller who arrives on an unintended floor must present the manifest at the nearest window and have it stamped on the reverse; unstamped manifests are confiscated and the traveller is charged a re-entry fee of two slips, payable in the office that issued it and not at the aperture itself.

"The aperture does not owe the traveller a destination. The Bureau owes the traveller a manifest."

— Bureau of Ambiguous Transit, Notice to Applicants, Annex 2

Aperture alpha and the People of the Long Corridor

Some apertures have been settled long enough to acquire residents. Aperture alpha, on the eastern arc of Floor 620, has had continuous occupancy since the Settlement of Aperture Alpha was recorded by clerk Ivessa Marrow, whose grievance — that she was never issued a manifest for the crossing she made each morning to reach her own desk — was upheld by the Registrar of Interior Complaints on the grounds that a repeated crossing is not a transit. Her ruling stands, though the Bureau of Ambiguous Transit has never accepted that it does, and continues to issue Marrow a manifest annually in a form it describes as ceremonial.

Scholarly dispute

The principal controversy concerns whether an aperture that opens onto itself constitutes a transit at all. Third Concordance, Annex 7, holds that passage requires "a departure and an arrival distinguishable in kind." The Eighth Concordance, Annex 7, drafted in the same scriptorium and sealed a fortnight later, holds that "the arrival shall be where the traveller is, and the departure where the traveller is not, and these need not differ." Both remain in force. The recorder of the Committee of Ambiguous Precedent notes in Registry of Ambiguous Precedent, Volume IX, that no case has yet been brought that satisfies one annex without offending the other, and that the Committee prefers to adjourn rather than choose.

Related but distinct phenomena are catalogued under Recursive Apertures and under Ambiguous Referents of a locative kind, which are held at the Aisle of Ambiguous Prepositions. Their vocabularies overlap, and the Bureau of Ambiguous Forms has twice petitioned to have the three fields consolidated; the petitions were filed under Identical Ambiguous Referents and have not been answered. Beneath the whole arrangement sits the standing caution of the Department of Ambiguous Metrics, repeated in every edition of its handbook: that what a surveyor records is not what a surveyor saw, and that the difference is the entire subject.