Chess. But the History of England has only one section, as our scheme
is developed, and thus the two might be said to be co-ordinated. The
apparent difficulty in such cases is entirely obviated by the use of a
fourth figure, giving nine sub-sections to any subject of sufficient
importance to warrant closer classification. In history where the
classification is made wholly by countries, a fourth figure is added to
give a division into _periods_. As the addition of each figure gives a
ten-fold division, any desired degree of minuteness may be secured in the
classing of special subjects. The apparent lack of co-ordination arises
from the fact that only the first three figures of these more important
heads are as yet printed, the fourth figure and the sub-sections being
supplied on the catalogues in manuscript. Should the growth of any of
these sub-sections warrant it, a fifth figure will be added, for the
scheme admits of expansion without limit.

The arrangement of headings has been sometimes modified to secure a
mnemonic aid in numbering and finding books without the Index. For
instance, the scheme is so arranged that China has always the number 1.
In Ancient History, it has the first section, 931: in Modern History,
under Asia, it has 951: in Philology, the Chinese language appears as
491. After the same manner the Indian number is 2; Egyptian, 4; English,
2; German, 3; French, 4; Italian, 5; Spanish, 6; European, 4; Asian, 5;
African, 6; North American, 7; South American, 8; and so for all the
divisions by languages or countries. The Italian 5, for instance, will
be noticed in 35, 55, 450, 755, 850, and 945. This mnemonic principle is
specially prominent in Philology and Literature and their divisions, and
in the _form_ distinctions used in the first 9 sections of each class.
Materials, Methods, or Theory occurring anywhere as a head, bears always
the number 1. Dictionaries and Cyclopedias, 3; Essays, 4; Periodicals, 5;
Associations, Institutions, and Societies, 6; Education, 7; Collections,
9. In the numerous cases where several minor heads have been grouped
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