(6) A second rule respecting the subdivision of a class is: "The constituent species must exclude each other." That is, the divisions or subclasses must not overlap. (See exception in Rule [8].)

Example: If a number of balls of several different materials, several different conformations, or constructions, several different colors, were to be divided into glass balls, hollow balls, and red balls, this rule would be violated, because some balls would be glass, hollow, and red.

(7) A third rule respecting subdivision is: "The divisions must be founded on one principle or basis." The application of this rule will generally form divisions that do not overlap. (See exception in Rule [8].)

Example: If a number of balls of several different constructions, several different materials, and several different colors were to be classified so as to provide a place for each kind of characteristic, they should be divided first, for example, according to construction into hollow balls and solid balls, each of these according to materials into glass balls, rubber balls, metal balls, wooden balls, etc., and each of the latter into red balls, blue balls, green balls, etc.

(8) When it is found that division into overlapping subclasses and on different characteristics is a lesser evil than an unwieldy number of subclasses that would otherwise result, then those subclasses based on characteristics deemed more important for purposes of search should precede in the list of subclasses those based upon characteristics deemed less important. (See Rule [6].)

(9) In arrangement of subclasses or subdivisions the miscellaneous groups containing material not falling within any of the specifically entitled subclasses, should stand first; those subclasses defined by effect or special use should precede those defined by function or general use; those containing matter that is related to the matter of other subclasses as whole to part should precede those subclasses that contain the part; and those defined by a characteristic deemed more important or significant for search purposes should precede those defined by characteristics deemed less important.

Whenever superior rank has been assigned to any selected characteristic by placing divisions based upon it in advance of divisions based[p. 28] upon other characteristics, this superiority should be maintained throughout.

Example: A partial schedule of Class 80 follows to illustrate the arrangement of subclasses:

Class 80.—Metal Rolling.

1.Miscellaneous.24.Die rolling—
2.Heating and rolling.25.Oscillating rolls.
3.Cutting and rolling.. .. . . . .
4.Drawing and rolling Mills—
5.Annular bodies.32.Coiling.
6.Screw threads—33.Work reversing.
7.Concave and roll.34.Three or more coacting rolls.
8.Platen rolling—35.Continuous—
9.Dies.36.Inclined trains.
10.Rods and wires.. .. . . . .
11.Tubes—41.Roll cooling and heating.
12.Idle rolls.42.Cooling beds.
13.Axial rolling.43.Feeding—
14.Segmental rolls.44.Tables.
15.Skelping.. .. . . . .
16.Wheels and disks.55.Housings.
17.Reworking56.Roll adjustments—
18.Concave and roll.57.Relief devices.
19.Platen and roll.58.Rolls—
20.Platen rolling—. .. . . . .
21.Disk platens.60.Processes—
22.Axial rolling—. .. . . . .
23.Pattern rolls.66.Flanged bars.