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Users want to see Thema presented as ONIX, especially to understand What do you do with a translator entry with a second subject or a qualifier? BISAC and Thema are very similar – both are designed to support trade retail sales and do that by naming a single subject as a "Main Subject" to be used by retailers. One way to think of that Main Subject is that it directs the book to the right retail buyer as most buyers work within subject areas. Librarians tend to work in a similar way with an interested professional specializing in areas. A Main Subject is like saying which book professional would care most about this book. There's a second special concern for Thema in that any subject code that applies to a book can be used (for example "slavery" applied to a work of Fiction) and the main subject would define if the book is fiction or nonfiction in such a case.
Otherwise, while the codes are hierarchical (example below), the actual book subjects are not given in a hierarchy. If the subject describes the book – be it a subject code or a qualifier or a regional code – you use it to describe the book. While both systems allow for as many codes as needed, BISAC codes seldom exceed three. As you'll see, Thema's schema is more granular and may require additional codes. In either case, focused and fewest will almost certainly give better retailer adoption than more volume.
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The US market (BISAC) determined that there was enough noir fiction being sold that wasn't associated with hard-boiled detective fiction to warrant its own code. Thema (international) may come to the same decision at a later date but as of writing has not. You should expect differences between the coding systems. (Note that these examples are based on older versions of BISAC and Thema and should be only used as example. The actual coding and their meaning may have changed).
Here is another example:
- Thema uses the general term Crime to cover "detective" but provides for "mysteries" in a variety of contexts, while BISAC has a specific heading for FICTION / Crime, but combines Mystery and Detective.
- Thema places True Crime as a minor heading under a hierarchy of Literature / Biography & non-fiction prose, while BISAC provides True Crime as a major heading with subdivisions for Espionage and so on.
- Therefore, to reproduce a code to replace a subdivision of BISAC's True Crime, Thema codes may require use of a secondary subject code (like the non-fiction code for "Espionage & Secret Services") to add that meaning to its True Crime subject.
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A book's CODES are hierarchical because retailers can take information from any part of the hierarchy, which makes the codes flexible. There's never a need to apply a high-level "fiction" code for a book once you've given it a any subject within the "fiction" list. The specific code contains all the information a retailer needs to know about its higher levels.
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