Floor Mu 44

Floor Mu 44 is a partial floor of the eastern spiral, occupying the interval between Floor 441 and Floor 449. It is not, strictly speaking, a floor of the Tower at all: it is the volume enclosed by the collapse of Floor 446, which came down in the winter of the year recorded as 6,112 and was never rebuilt. The Librarians elected to catalogue the void rather than the ruin, and the designation Mu was assigned by the Bureau of Unfinished Floors to mark an aperture that has been given a number but no structure. Visitors should note that the eastern stair does not stop at Mu 44 and that walking it is done at the walker's own risk.

Origin and the Collapse of 6,112

Floor 446 was a storage floor for redundant shelving. Its north wall had been noted as unsound in the survey of 5,998, and the note was filed, as was customary, in the floor it described. When the wall failed, the floor above it descended through Floor 446, Floor 445 and the eastern landing, coming to rest at last on the reinforced deck of Floor 441. No one was killed. Four porters were unaccounted for until the following Tuesday, when they were found on Floor 449 having walked down through the rubble without noticing that they had left the stair.

The Concordance of Loads, Article 12, holds that a collapsed floor retains its number until such time as the number is formally surrendered. Floor 446 has never surrendered its number. Mu 44 therefore does not replace it; the two designations coexist, and correspondence addressed to either will reach the same vacant post.

Measurement and the Mu Standard

The floor's principal function is metrological. Because the void has a definite boundary — the underside of Floor 449, the deck of Floor 441, and the inward faces of the surviving eastern walls — it can be measured, and it is the only space in the spiral whose measurements are agreed. The Mu Standard, ratified by the Sixth Recension in the year recorded as 6,140, fixes the height of a standard storey by reference to this void and to nothing else.

"A storey shall be the distance between the upper surface of a floor and the upper surface of the floor next above it, save where the floor next above has fallen, in which case the distance shall be taken as it stands in the aperture of Floor Mu 44, which does not move."

— Sixth Recension, Annex 2, clause 4

The clause is widely admired and widely evaded. The Office of Vertical Assay maintains that the Mu Standard is a standard of rubble and that its use for standing floors is an abuse; the Bureau of Unfinished Floors replies that all floors are falling, only at different rates. The dispute is now in its fourth century and has produced eleven annexes, none of which alter the measurement.

Dispute Between the Surveyor and the Porter

The floor takes its standing in the cataloguers' quarrels from the grievance of Ennet Vole, a porter of the eastern spiral, who in 6,151 refused to carry a load of ledgers through the aperture on the grounds that the floor did not exist and could not therefore be walked. Vole was docked two shifts. He appealed under the Concordance of Passage, Article 3, which provides that no porter may be required to traverse a floor that appears in no inventory.

The Bureau of Unfinished Floors produced its inventory. Mu 44 appeared in it, with a number, a name and an extent in the Mu Standard. Vole replied that the inventory listed the hole and not the floor, and that the hole could be walked only by falling. The Court of Vertical Appeals found for the Bureau on the narrow ground that the inventory had been correctly filed, and against the Bureau on the broader ground that Vole was owed his two shifts. Both findings are recited in the standard commentary and neither has settled anything. Vole returned to work and thereafter carried his loads along the western arc, which took him forty minutes longer and which no one has since thought worth contesting.

Use of the Aperture

The aperture is used for two purposes. The first is calibration of plumb lines, for which its unusual stillness recommends it: the eastern spiral is sheltered from the draughts that trouble the outer floors, and a line dropped through Mu 44 hangs true to a tolerance of one part in nine thousand. The second is the disposal of failed recensions. Copies of superseded Concordances are unbound, weighted and dropped through the aperture, and are understood to come to rest somewhere in the mass beneath Floor 441. No attempt is made to recover them.

Small items are strictly prohibited from the eastern arc between Floor 441 and Floor 449, and any object dropped must be reported to the floor warden before the bell. A floor warden is appointed to Mu 44 once every nine years, serves from Floor 441, and has never once been required to attend in person.

Recorded Confusion

The floor is frequently confused with the adjacent reversal floor Floor π and, less forgivably, with the custodial floors about Floor 606. The confusion is compounded by the Concordance of Nomenclature, which forbids the use of Greek letters as floor designations except where the floor is already designated, an exception drafted for exactly one floor and never since applied to any other. The Office of Vertical Assay has twice recommended that Mu 44 be struck from the roll and its void re-absorbed into Floor 446, and twice the recommendation has been filed in Floor 188, where such recommendations go.