(1) That will probably be first looked under by the class of people who
use the library;
(2) That is consistent with other entries, so that one principle can
cover all;
(3) That will mass entries least in places where it is difficult to so
arrange them that they can be readily found, as under names of nations
and cities.
This applies very slightly to entries under first words, because it is
easy and sufficient to arrange them by the alphabet.