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Subject categories: Thema and choosing a shelf
Every book is filed somewhere before a reader sees it. Subject codes decide where, and choosing them well is a small task with a long reach.
Before a reader finds a book, a system has already decided where it belongs. In a physical shop, that means a shelf. Online, it means a category page, a browse path and a set of search filters. In both cases, the decision starts with subject codes supplied by the publisher.
Those codes are easy to treat as a form to fill in. They deserve more thought than that.
What Thema is
Thema is the international subject classification scheme used by the book trade. It is maintained by EDItEUR, the organisation that also looks after other book-trade data standards, and it was designed to work across countries and languages so that a single set of codes can travel with a book wherever it is sold. It was developed to replace national schemes such as BIC in the UK, and other countries have adopted it alongside or in place of their own systems.
A Thema subject code describes what a book is about or what kind of book it is. The codes are arranged in a hierarchy, from broad headings down to narrow ones. Fiction, for example, divides into genres, and genres divide further.
Thema also has qualifiers, which add information that the subject code alone cannot carry. There are qualifiers for place, language, time period, educational purpose, interest age and style. A crime novel set in 1950s Glasgow can be coded as crime fiction, then qualified with Scotland, Glasgow and the relevant decade. A reader browsing Scottish fiction, or fiction set in that period, then has a way to reach it.
How codes are used
Retailers, wholesalers and libraries take the codes supplied with a book's metadata and map them onto their own categories. Each organisation's mapping is different, and some use other schemes as well, such as BISAC in the United States. The subject codes the publisher supplies are the raw material they start from.
Because of this, the codes influence where a book appears in browse categories, how bookshops think about shelving it, and sometimes which promotions it is eligible for. They also feed library catalogues, where subject access matters to how readers find books.
Choosing well
Start with the main subject, and be precise. The main code should be the most specific one that is accurate. A historical romance set in Regency England is better coded as historical romance than as romance, and better still if its setting is added with qualifiers.
Add secondary codes only where they are genuinely true. It is tempting to add categories that seem busy or fashionable, but a book filed in the wrong place disappoints the readers who find it there, and disappointed readers tend not to buy.
Think about the reader, not only the content. A book about grief might sit in psychology, in memoir or in self-help, depending on how it is written and whom it is written for. The question is where the reader who wants this book will look.
Look at comparable books. Seeing how similar titles are categorised by retailers is a useful check, though it tells you about their mappings rather than the Thema codes behind them.
Use the qualifiers. They are the part of Thema most often left blank, and they are where much of its precision lies.
Keeping them current
Codes are not fixed forever. If a book finds a different readership from the one expected, revising its subject codes can move it closer to that readership. Changes take time to flow through to every retailer, so they are best made deliberately rather than often.
A good subject code does not sell a book. It puts the book where the right reader is already looking, which is most of what anyone can ask of it.