Chapters, bookmarks, resume-where-you-left-off. M4B is the audiobook container format, and anyone who has organised a library knows the extension on sight. That gives this three-character domain a built-in audience with unambiguous search intent. A conversion tool, a library manager, a player, a self-publishing service, or a podcast-to-audiobook product would all be found by people who already know what the letters mean. The market supports it. Audiobooks have grown steadily for a decade while the tooling around them stayed thin, particularly for independent authors producing their own, which leaves room for products and a strong front door for one.

Two readings landing on the same extension without conflict, which is unusual and useful. Context resolves it instantly while the buyer pool doubles. As an initialism, M4B is the audiobook file format, giving it currency with anyone in spoken-word publishing. On the .org that suits an open library, an accessibility project, or a nonprofit producing audiobooks for people who can’t read print, which is a real sector with real institutional funding.