Threshold Phenomena

Threshold Phenomena is the collective designation, under Concordance 14.1.02, Annex B, for all observed effects arising at, on, or within one half-span of a recognized threshold of the Tower. The category is deliberately broad. It includes phenomena that occur while crossing, phenomena that occur while declining to cross, and a residual class of phenomena that occur to persons who have never approached the threshold in question and do not know that one exists. The Office of the Clerk of the Threshold maintains the working inventory, presently in its forty-first recension, and has never claimed it complete.

Threshold Phenomena are distinct from, and frequently confused with, Threshold Grace, which is a dispensation and not an effect. A phenomenon that merely resembles grace is nonetheless catalogued as a phenomenon until authenticated, and authentication may take decades. The distinction matters chiefly at audit.

Scope and classification

An effect qualifies for the inventory if it is reproducible, occurs within the half-span, and can be described without recourse to intent. The last condition excludes a great deal. Doors that open for one person and not another are recorded as ordinary; doors that open for one person and not another and report having done so are recorded under Class IV.

The classes run from I to VII. Classes I through III concern light, sound, and apparent distance, and are the province of the Luminance Threshold clerks and the Attenuation Threshold survey. Class IV concerns testimony given by the threshold itself, which the Office accepts in writing but not under oath. Class V concerns duplication of persons at the line, and is rare enough that the inventory still cites the original 1,102 entry by floor and name. Classes VI and VII concern, respectively, phenomena that persist after the threshold is removed, and phenomena that persist in a floor where no threshold was ever cut.

The half-span is measured from the sill, not the lintel. This was settled in the winter of 1,107 after eleven years of dispute between the surveyors of the Threshold Table and the clerks of the Unspoken Threshold, who held to the lintel. The surveyors prevailed on the grounds that the sill can be found and the lintel sometimes cannot. The Unspoken clerks have entered a standing dissent, renewed every ninth year at replacement of the Table, which the Office files without reading.

The Kestrel Recension

The inventory's third recension, prepared under the Keeper of the Threshold Marrow Kestrel, introduced the practice of recording phenomena by the reported sensation rather than the observed cause. This reversed two centuries of method and has never been fully undone. Kestrel's reasoning survives in her preface:

"The cause of a threshold phenomenon is at the threshold, which is to say elsewhere. The sensation is in the traveller, which is to say here. I propose we catalogue what is here."

— Marrow Kestrel, Preface to the Third Recension of the Threshold Phenomena Inventory

The Kestrel method produced an inventory that was immediately useful and permanently ambiguous. A single step at the Ninety-Eighth Landing is entered four times: once as a pressure on the ears, once as a smell of wet stone, once as the conviction of having been preceded, and once as nothing at all. The fourth entry is cross-referenced to the other three and marked contingent.

Concordance dispute

Considerable scholarly disagreement attaches to the threshold of reportability. Concordance 18.2.77 sets a more lenient threshold for ephemeral phenomena than Concordance 892's definition of a Definitively Lost Echo, with the consequence that an echo lost at a threshold may be Classifiable Auditory Event under one instrument and no event whatsoever under the other. The Lament Marble Interface was drafted to reconcile the two by fixing a standardized measurement, and succeeded at every floor except those where both instruments are cited in the same complaint.

Concordance 14.1.02 itself declines to resolve the matter, noting at Annex B, clause 9, that "where two thresholds of reportability obtain, the phenomenon shall be recorded twice and reconciled by a later age." No later age has yet taken up the file. The Register of Minor Architectural Behaviors disclaims jurisdiction over all of it, holding itself to phenomena below the threshold of the Threshold Phenomena inventory — a boundary that the Register admits it cannot locate, since locating it would place it above.

Notable entries

The inventory's most cited entry remains 1,044: the Vance Threshold incident, in which a survey party of six crossed a marked line on the Forty-Third Spiral and arrived as five. The party's own count agreed with the census. The discrepancy was resolved in favour of the census by the simple expedient of not asking the party again. Entry 1,044 has generated thirty-one learned commentaries and no additional members.

Entry 2,208, the Provisional Threshold, is included despite the provisional marking because the triangle has now stood for four hundred years and its effects show no sign of lapsing. The Office notes that provisional status is a matter of the marking, not of the passage of time. It further notes that the yellow paint is renewed every third year by the Vanguard of the Unseen Threshold, and that this is the only maintenance the Office does not schedule itself.

Entry 3,017 is a flight of stairs on the Eleventh Archive Landing that produces a mild disorientation in persons carrying more than three books. It has been investigated twice and closed twice. The closing memorandum of Surveyor Odile Fram records that the effect ceased when the books were set down, and recommends that the inventory "take no further interest, as the phenomenon is indistinguishable from the ordinary difficulty of stairs."

Measurement and maintenance

Phenomena are measured against the Inertia Threshold (θ), which grades the resistance of a phenomenon to reclassification. A high θ indicates an entry that will not be moved once filed; a low θ indicates one likely to drift into another class before the next recension. The Office regards a drift as a failure of drafting and not of the phenomenon, and does not revise the phenomenon.

Crossing the inventory line while carrying a lit lamp is permitted. Crossing it while carrying the inventory is not.

The Threshold Mirrors are maintained at four crossings for the detection of Class V duplication and are polished twice yearly. All other inspection is by report. The Office has stated, in its standing preface, that a phenomenon unobserved is not thereby absent, and that the archive's silence on a given landing should be read as an absence of clerks rather than an absence of phenomena.