Chapter Markers
A chapter marker is a named time offset stored inside an audiobook file, so a player can show a chapter list you can jump between. "Chapter 7, The Orchard Changes Hands, at 4:12:38" is one marker. Without them, a ten-hour audiobook is a single scrub bar and finding where you left off means guessing.
Which formats carry them
M4B stores markers natively, which is the main reason it exists as a distinct extension. MP3 has no standard chapter mechanism in practice, so distributors that require MP3 ask for one file per chapter instead, and the filename does the job the marker would have done. That is why ACX wants a separate file per section rather than one long recording: the file boundaries are the chapter structure.
Where they actually come from
Markers are not generated from the audio. Nothing listens to a recording and detects that a new chapter started. They are written from the structure of the manuscript, which means the chapter structure has to survive the whole way from your document to the finished file.
This is where it usually breaks. A manuscript can look perfectly chapter-broken to a human and carry no structure at all underneath:
- Real heading styles (Heading 1, Heading 2) are machine-readable. A parser can find every chapter reliably.
- Manual formatting that looks like a heading, bold text at 18pt, centred, with blank lines above it, is invisible to a parser. It is body text that happens to look large.
- Page breaks alone mark where a chapter starts visually and say nothing about what it is called.
A manuscript in the second or third category produces an audiobook with either no markers or wrong ones, and the fix is upstream in the document, not in the audio.
What to check before uploading
Open the navigation or document map pane in your word processor. If your chapters appear there as a list, the structure is real and will carry through. If the pane is empty, your chapters are visual only. Applying Heading 1 to each chapter title takes a few minutes and is the single highest-value piece of manuscript preparation you can do.
Why they matter commercially
Chapter navigation is one of the few things a listener notices immediately when it is missing, and it is a common subject of one-star reviews on otherwise well-produced audiobooks. It also affects nothing about audio quality, so it is entirely a production-process question rather than a recording one.