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When copy cataloging, I've noticed that some LC records for books on legal topics have two DDC numbers (for example, 343.04 and 342.34). Why is this, and which one should I use?
When DDC Edition 20 was still being used, the Library of Congress Decimal Classification Division made one exception to its practice of eschewing optional numbers: it supplied both the preferred number and the option B number for works in the field of law. Starting with DDC 21, the Decimal Classification Division no longer supplies option B numbers. The instructions for creating option B numbers are still found in DDC 21 for libraries wishing to use option B, but those libraries must build their own option B numbers.
Meanwhile, in bibliographic records with Edition 20 numbers, the first DDC number is the preferred number, and the second number is the option B number. In a record like the one you are referring to, 342.34 is the option B number. In DDC 20, the main instructions for option B appeared at 340; in DDC 21, they appear at 342-349; but the instructions themselves are the same. In this case, the option B number was created following the instruction found at 342 for comparative law and law without jurisdiction (the instruction is the same in both DDC 20 and DDC 21):
"(If Option B under 342-349 is chosen, class here comparative law and law without jurisdiction by adding to base number 342 the numbers following 34 in 342-348, but omitting the first 0 after decimal point, e.g., comparative criminal procedure 342.55 [not 342.505] )"
The number 342.34 was built by adding to base number 342 the numbers following 34 in 343.04 (that is, 304), but omitting the first 0 after the decimal point (that is, 34). Here is the summary breakdown:
| 342 | Optional use of this number for comparative law and law without jurisdiction | |
| 34 | 343.04 | Tax law |