Home → Cataloging Bibliographic Records → Merging Bib Records → 020$z Versus 020$a
Last Updated 03/06/2026
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To avoid accidental merging of records for items in different formats, catalogers should utilize subfield $z in the =020 MARC field for certain records instead of subfield $a. Subfield $z is appropriate for cancelled or invalid ISBNs and when the item significantly differs from its original counterpart. Examples include items in kits, audio-enabled books that share ISBNs with print versions, and multi-volume sets. Video identifiers, such as UPCs, should similarly follow this principle, with UPCs for split combo packs recorded in =024$z to prevent mismatches during batch imports.
To prevent accidental matching and merging of records that should remain separate, catalogers should use a subfield $z in the =020 MARC field rather than a subfield $a for certain types of records. The subfield $z is typically used for cancelled or invalid ISBNs. Additionally, catalogers should use subfield $z when the item in hand differs in a significant way from the original item that the ISBN was attached to. Specific common examples of this type of material include:
The subfield $z helps to prevent matching and merging of physically different materials, but you are still able to perform a search for an ISBN in a subfield $z in the catalog and Z39.50 interface, so you do not lose any important search functionality if the ISBN is in a subfield $z.
Occasionally you may come across a book that shares the same ISBN with an entirely different book. Some publishers will "recycle" ISBNs. This is common with small organizations, self-publications, and print-on-demand/public domain publishers. If there is already an existing record for a non-matching resource with the same ISBN in NC Cardinal, catalogers should place the ISBN for the item in hand in the =020$z to prevent the records from being merged.
You can find more information about working with duplicate ISBNs in Appendix E: 020 Tag (ISBNs).
Kits often contain a book with an ISBN, or another material that may have a standard identifier. However, a record for a single book should not be matched and merged with a record for a kit, even if that kit includes the book in question as one of its component pieces. To avoid this, an ISBNs associated with any books included as component pieces of the kit should be listed in subfield $z rather than $a. This will prevent the kit record from being merged with the stand-alone bookrecord, while still allowing the kit to appear in search results for the ISBN.
You can find more information about cataloging kits in the Kits page, of the Cataloging Specialized Item Types Knowledge Book.
Audio-enabled books such as VOX Books and Wonderbooks often share the same ISBN with the standard versions of their respective books, because the audio device is attached to an existing printed copy of the book. If the ISBN for the audio-enabled book matches the ISBN of a print copy of the book, it should be listed in the =020$z in the bib record for the audio-enabled book. The record for the original, standard version of the book should include the ISBN in the =020$a. This will help prevent the records for the different versions from being matched and merged.
You can find more information about cataloging kits in the Wonderbooks and VOX Audio-Enabled Books page of the Cataloging Specialized Item Types Knowledge Book.
Note: If there is not yet a bib record for the print version of the book, catalogers should still include the ISBN for an Audio-enable book in the subfield$z to prevent merging if a future cataloger attempts to import a record for a print version.
Multi-volume sets, such as encyclopedias, may one ISBN for the full set, with a seperate ISBN for each individual individual volume. When cataloging these materials, catalogers should include the ISBN for the set in a subfield $a, and the ISBNs for the individual volumes in subfield $z's. Additionally, including a subfield $q identifies which ISBN is for which item.
=020 \\$a9781414487366$q(set)
=020 \\$a1414487363$q(set)
=020 \\$z9781414487373$q(Vol. 1)
=020 \\$z1414487371$q(Vol. 1)
=020 \\$z9781414487380$q(Vol. 2)
=020 \\$z141448738X$q(Vol. 2)
If a library has only one volume of a multivolume set or has chosen to catalog each individual volume as its own separate recor, the cataloger should put the ISBN for the individual volume in a subfield $a, and the ISBN for the complete set in subfield $z. Including a subfield $q helps to denote which ISBN goes to which item in this situation as well.
=020 \\$a9780028974316$q(Vol. 1)
=020 \\$a002897431X$q(Vol. 1)
=020 \\$z9780028974392$q(set)
=020 \\$z0028974395$q(set)
Video standard identifiers, UPCs, are listed in the =024 MARC field, and should follow the same principles as listed above for the 020 field. When cataloging individual discs from a multi-format combo pack, the two items must be cataloged on separate bibliographic records using the DVD only, Blu-ray only, and 4K Ultra HD Blu-ray only MARC templates. In order to avoid incorrect matching when other libraries are batch importing, the UPC for split combo packs should be included in the =024$z, not hte =024$a.
In addition to the subfield $z, catalogers should include both a subfield $q and a =500$a, noting that the disc was originally part of a set.
Suggested language for the =500$a: "Originally released as part of a Blu-ray + DVD combo pack" or "Originally released as part of a Blu-ray + 4K Ultra HD Blu-ray combo pack."
You can find more information about cataloging Video Combo Packs the Videos page of the Cataloging Specialized Item Types Knowledge Book.