An essential component of a seven-dimensional library classification system discovered by Archchancellor Scrubbs. Apparently stands in as close a relationship to slood as Space does to Time, or Matter to Energy. It's as clear as daylight, if you remember to keep your eye on the blit. The lads at Brazeneck College have apparently come up with a new algorithm for handling wave-spaces in higher-level blit, which is measured in Drinkies.
Library classification is more difficult than you think at first glance. Take what looks to be a nice simple concept, like cabbages.
Books about all mundane and prosaic aspects of the vegetable are shelved under Brassica and coded (blit) UUSSFY890-9046.
Although, obviously, Mr Cauliflower's Big Adventure is better shelved as UUSS J3.2 (>blit) 9 and The Tau of Cabbage will reference to UUSS (blit+)60-sp5-09--hl (blit) while A Tourist's Guide to Big Cabbage would logically go to UUSSTR (CC) (>blit#) 780-225.
Simple and foolproof, isn't it?
In The Science of Discworld IV: Judgement Day, reference is made to the Bliss system of library classification - thought in some respects to be better than the far better known Dewey Decimal, but forced into a footnote to history by a technically inferior system which, like VHS tape cassettes, had better marketing men and superior PR than Betamax. In a Discworld context, think of the Blit system.