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Glossary

What is non-linear content in EPUB?

Updated:

Non-linear content is a document listed in the EPUB spine with linear="no": auxiliary material, such as notes, answer keys or descriptions, that is outside the main reading order. Reading systems may skip it while paging, so it has to be reachable from a link.

Why it matters when you convert a PDF to EPUB

A converter that moves endnotes, an answer key or an appendix of figures into separate files has to decide whether readers page through them or reach them only through links. Marking such a file non-linear without a link to it leaves it in the book but impossible to open on some readers.

EPUB 3.3 describes linear="no" as a hint, not an instruction: a reading system may still show the document where it sits in the spine. EPUBCheck requires a hyperlink to every non-linear document (OPF-096) and at least one linear item in the spine (OPF-033).

How EpubCC handles it: every chapter it writes is linear and footnotes stay inside their chapter, so its EPUBs contain no non-linear documents.

Related terms

EPUBCheck messages about this

  • OPF-096 Non-linear content must be reachable, but found no hyperlink to "…".
  • OPF-033 The spine contains no linear resources.

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FAQ

What is non-linear content in EPUB?

Non-linear content is a document listed in the EPUB spine with linear="no": auxiliary material, such as notes, answer keys or descriptions, that is outside the main reading order. Reading systems may skip it while paging, so it has to be reachable from a link.