ule for cross-referencing where several inventions
are claimed bearing to each other any of the relationships indicated
above, cross-referencing being necessary in one direction only where the
matter illustrated is coextensive with the matter claimed.

(5) It definitely limits the field of search for any _unitary invention_
in any class so arranged, as no patented invention having the
limitations imposed by a unitary claim should be found in any subclass
below the subclass properly defined to receive it or those indented
under it. Parts of such inventions may be found below or following this
subclass in the same class if these parts are within the class
definition, or elsewhere in the useful arts if not within that
definition. The unitary invention may be found in the subclass limited
to it and certain subclasses arranged _above_ or _before_ it adapted to
receive organizations of which it may be a part.

A complete system of arrangement should comprise (1) a display of the
entire field of the useful arts in a manner to show the relation of the
larger as well as of the smaller groups,--carrying the appropriate
relationship as far as possible from the highest genera to the lowest
species, the arrangement being such as would bring materials most nearly
alike into closest propinquity regardless of the names they may be
called by. (2) Supplementary to this classification arrangement by ideas
there should be an alphabetical index of subclass titles, appropriately
cross indexed, and additional titles of various technical and trade
names of things classified under subclass titles.

DEFINITION.

Definition is indispensable in any classification and is very
difficult. Every class must be defined and all of the groups under it.
After definitions have been made and printed, they are sometimes found
inadequate and must be supplemented by the definitions of other classes.
This is unavoidable while the complete material remains unexplored.
Definition in the strict logical sense is not to be expected, nor is it
necessary. It is commonly sufficient if an explanation or comparison be
made sufficient to direct the mind to the character of the contents of
the group and indicate its limitations. Hitherto four of the five
predicables of ancient logic have been mentioned, to wit, genus,
species, property, and accident. In connection with definition, the
fifth predicable, difference, is useful. To define a class, it is
sufficient, genera