together under the head Other, it always bears the number 9. Wherever
practicable, this principle is carried out in sub-dividing the sections.
For instance, the Geology of North America, which bears the number 557 is
sub-divided by adding the _sections_ of 970 (History of North America).
The Geology of Mexico then bears the number 5578: mnemonically, the first
5 is the Science number; the second 5, Geology; the 7, North America; and
the 8, Mexico. Any library attendant or reader after using the scheme a
short time will recognize at a glance, any catalogue or ledger entry,
book or pamphlet, marked 5578 as something on the Geology of Mexico.
Users of the scheme will notice this mnemonic principle in several
hundred places in the classification, and will find it of great practical
utility in numbering and finding books without the aid of Catalogue or
Index, and in determining the character of any book simply from its call
number as recorded on the book, on all its catalogue and cross reference
cards, on the ledger, and in the check box.

In naming the headings, brevity has been secured in many cases at the
sacrifice of exactness. It was thought more important to have short,
familiar titles for the headings than that the names given should express
with fullness and exactness the character of all books catalogued under
them. Many subjects, apparently omitted, will be found in the Index,
assigned, with allied subjects, to a heading which bears the name of the
most important only. Reference to this Subject Index will decide at once
any doubtful points.

In arranging books in the classification, as in filling out the scheme,
practical usefulness has been esteemed the most important thing. The
effort has been to put each book under the subject to the student of
which it would be most useful. The content or the real subject of which
a book treats, and not the form or the accidental wording of the title,