Section 5.7 of the DDC Introduction indicates that a work on two subjects should be classed with the subject receiving fuller treatment. But what about works in which the two subjects receive equal treatment? In this case we invoke the first-of-two rule—but only if the rule of application, which takes precedence over other rules, doesn’t apply. This rule instructs us to class such a work in the number coming first in the DDC schedules.Note that the first-of two rule does not instruct us to class the work in the number of the subject coming first in the title of the work. Such an interpretation would make collocation an issue of whether all works on the two-subjects-treated-equally happened to name them in the same order in the title. The first-of-two-rule also does not instruct us to class the work in the number of the subject coming first in the alphabet. This would make collocation an issue of whether all languages name the two subjects so that they stand in a single fixed order. In contrast, using the schedules as the basis for the first-of-two rule provides a mechanism whereby two-subjects-treated-equally will consistently be classed in the same number.
For example, given the first-of-two rule, works giving equal treatment to 782.26 Motets and 782.294 Psalms should be classed in the first of the two numbers, that is, in 782.26. And, given the first-of-two rule, works giving equal treatment to 746.422 Knotting and 746.436 Tatting should be classed in the first of the two numbers, that is, in 746.422.
Of course a work giving equal treatment to knotting and tatting is as relevant to tatting as it is to knotting. Statements throughout this post about the number that the first-of-two rule instructs us to use should be understood to address the assignment of the standard DDC number, captured in the 082 field (Dewey Decimal Classification Number) of the MARC bibliographic record. The number not chosen as the standard number should be considered for inclusion in the 083 field (Additional Dewey Decimal Classification Number). Our work on knotting and tatting might then have the following fields added to its bibliographic record:
082 #4 $a 746.422 $2 23
083 #4 $a 746.436 $2 23
while our work on motets and psalms might have the following fields assigned:
082 #4 $a 782.26 $2 23
083 #4 $a 782.294 $2 23
As we saw in a follow-up post on the rule of application, sometimes the basic rules that govern the assignment of numbers in the DDC are built into its notes. For example, consider a work on abstracting (025.41 Abstracting) and subject indexing (025.47 Subject indexing and cataloging), which gives the two subjects equal treatment. By the first-of-two rule, such a work should be classed in the first of the two numbers, that is, in 025.41. Application of the rule is reinforced in this context by the class-here note at 025.41, which states that comprehensive works on abstracting and subject indexing are to be classed in that number. Another example can be found at 382.093–382.099 International commerce in specific continents, countries, localities. Provision is made here for representing commerce between two geographic units by adding Table 2 notation for both of them, separated by an intervening 0. If the work emphasizes one of the geographic units, its Table 2 notation is given first. But “if emphasis is equal, give priority to the one coming first in Table 2,” which echoes the first-of-two rule.Sometimes, however, notes are given that override the first-of-two rule. Consider, for example, a work giving equal treatment to American literature (which is classed in 810 American literature in English) and British literature (which is classed in 820 English and Old English literatures). By the first-of-two rule, such a work would be classed in the first of the two numbers, i.e., 810. But at 810 we find the class-elsewhere note, “Class comprehensive works on American literature in English and English literature in 820.” Another example arises with works giving equal treatment to historical bibliography, which is found in the class-here note of 002 The book, and analytical bibliography, which is classed in 010.42 Analytical bibliography (Descriptive bibliography). By the first-of-two rule, such a work would be classed in the first of the two numbers, i.e., 002. But at 002 we find the class-elsewhere note, “Class comprehensive works on historical and analytical bibliography in 010.42.”
An exception to the first-of-two rule comes into play when the two subjects are the two major subdivisions of a broader subject, in which case a work giving equal treatment to the two subjects is to be classed in the number for the broader subject of which they are major subdivisions. For example, a work giving equal treatment to dialectology (417.2) and historical linguistics (417.7) should be classed in 417 Dialectology and historical linguistics (and not in 417.2), because 417.2 and 417.7 are the two major (and indeed, the only) subdivisions of 417.
Finally, what may seem like a subtle distinction needs to be drawn between treatment of two subjects, to which the first-of-two rule may apply, and two aspects of a single subject, to which preference notes and preference tables apply. We have seen that classification of a work giving equal treatment to the two subjects, knotting and tatting, is governed by the first-of-two rule. Consider by way of contrast a work on knotted rugs, a single subject with two aspects. The one aspect is classed in 746.422 Knotting; the other aspect is classed in 746.7 Rugs. If the first-of-two-rule applied (but it doesn’t), the work would be classed in 746.422, but the preference table under 746.1–746.9 gives priority to 746.7 Rugs over 746.4 Needlework and handwork.