Throughout the catalogues the number of a book shows not only _where_
it is but _what_ it is. On the library accounts the character of each
person's reading is clearly indicated by the numbers charged, and the
minutest statistics of circulation in any subject are made by simply
counting the call slips in the check box, and recording the number
against the class number in the record.

By the use of size numbers the greatest possible economy of space may be
secured, for the size distinction may be made for every inch or even less
if desired, and this without additional labor, as it will be seen that
the size figure, when introduced, requires one less figure in the book
number, and so does not increase the number of digits as would at first
appear.

Parts of sets, and books on the same or allied subjects, are never
separated as they are sure to be, sooner or later, in every library
arranged on the common plan, unless it be frequently re-arranged and
re-catalogued. The great expense of this re-cataloguing makes it
impracticable except for a few very wealthy libraries. In this system
the catalogue and book numbers remain unchanged through all changes of
shelving, buildings, or arrangement. In addition to its own peculiar
merits, this plan has all the advantages of the card catalogue principle
and of the relative location, which have been used and very strongly
approved by prominent libraries.

As in the card catalogue system, there is room for indefinite expansion
without devices or provisions. Space is the only requisite and if the
shelf room is exhausted, the floor space is equally good, except for the
inconvenience of stooping.