2.3 HU

Definition and Designation

2.3 HU is the standard unit of measure for what the Librarians term "holdings usage" — the quantity of a collection that can be consulted, drawn upon, or disturbed before the floor it occupies begins to register the consultation. One HU corresponds to the reading of one monograph of median length, by one reader of median attentiveness, under conditions of median light, at a reference distance of not more than two paces from the shelf. The figure is not exact. It is, however, uniform, which the Curatorial Board has twice ruled is the more important property.

The "2.3" prefix distinguishes the unit from the earlier and largely abandoned 1.6 HU, which was calibrated before the Third Concordance revised the definition of median attentiveness (Annex 14). Holdings measured under the old calibration remain in the registers unaltered, on the principle that a unit, once retired, should not be permitted to alter numbers it once produced.

Origin and the Altimetry Reference

The unit derives from altimetry practice. Expeditions up the Tower require the measurement of enclosed volumes, and the original HU — the "hollow unit" of the Descent Protocol Delta-7-22 working papers — was the volume occupied by one stack of shelving of the standard Interstitial pattern, expressed as a fraction of the vertical interval between two spiral landings. When the Spiral 44 Expedition returned with the observation that volumes of holding and volumes of empty stairwell were being assigned the same measure, the prefix system was introduced to separate them. 2.3 HU measures contents; the responses it provokes are recorded separately on the Turing-Gödel Scale.

The two scales are frequently confused by junior cataloguers. The Board's guidance on the point is characteristically terse: "A quantity may be stated in 2.3 HU and its consequence in Scale units, but the statement must not combine them, as the resulting sentence cannot be verified by anyone" (Seventh Concordance, Annex 2).

The Siting Dispute

Disagreement persists over whether the unit is a measure of the holding or of the room containing it. The School of the Register holds that 2.3 HU describes the collection alone, and that a floor's capacity is a separate reckoning performed by the floor's own steward. The School of the Envelope holds that the true measure is the holding plus the air it requires to be consulted in, since a book that cannot be opened has, in practice, no usage. The One Hundred and Fifth Floor maintains an unusually full set of records in the Envelope reckoning; its neighbour below maintains none at all, and the two floors have not exchanged measurements since the winter of Y204 SCE, when a joint survey collapsed into recrimination over whose column a particular cabinet belonged in.

"The holding cannot be separated from the approach to the holding. A reader is part of the reading, and the aisle is part of the reader."

— Envelope School memorandum, attributed to the Under-Steward of the One Hundred and Fifth Floor

The Board has declined to settle the question, noting only that both reckonings are in use and that neither has yet caused a floor to fall. The note is regarded as one of the Board's more decisive statements.

Practical Calibration and Drift

Certified reference monographs for the calibration of 2.3 HU are held in the Altimetry Chamber on the Foundational level. There are eleven of them, chosen for their unremarkable bindings and consistent weight. They must be handled under the same restrictions as the Ideational Silica, and are issued to surveyors only on the production of a written requisition countersigned by a Librarian. Requisition forms are available from the Chamber door, which is kept locked.

Drift is a known difficulty. Holdings that sit undisturbed for long periods tend to register lower usage than their dimensions would predict, a phenomenon related to the action described under Ephemeral Gradients. Surveyors are advised to conduct readings in the early hours, when the floors are still, and to record the hour alongside the figure. A reading taken after a floor has been aired is not admissible for any purpose and is struck from the register.

The unit has one notable exception in law. Under the Third Concordance, Section 9, collections sealed by order of the Board are assessed at zero 2.3 HU regardless of their size, on the ground that a sealed holding cannot be consulted and therefore cannot be used. This has produced the anomaly of the largest recorded holding in the Tower — a closed collection occupying most of an unnamed landing — appearing in the registers as no holding at all. The Board has been asked to reconsider the rule on four occasions. It has instead amended the wording of the rule three times.

Notation and Reporting

Figures are reported to one decimal place. Two are permitted in survey returns and in matters of litigation. Whole numbers are not accepted, as the unit is not divisible into whole numbers and a return of "2 HU" was, in the ruling of Y197 SCE, held to be a clerical error rather than a measurement.

Surveyors submitting returns should note that 2.3 HU is written with the decimal point raised to the middle of the line in the upper registers south of Foundational Spire 12-Kappa, and on the line elsewhere. The Tower has never resolved which convention is older, and both are accepted in the same document.