The OPS folder (or OEBPS if you prefer, the two names are interchangeable) contains the guts of your EPUB. OEBPS stands for ‘Open eBook Publication Structure’ and OPS stands for ‘Open Publication Structure’. OEBPS is technically a predecessor of OPS, but both naming conventions are still in use.
Whether you call it the OEBPS or OPS folder, it’s where you will find the content of the book, plus any fonts, images or audio/visual materials contained within the book. It also contains your CSS stylesheet, and the OPF file.
Folders
Certain types of content should be organized within their own folders inside an EPUB. Images, fonts, and CSS all typically get their own folders, and if your book has audio or video elements, those would get their own folders too.
CSS
The CSS (Cascading Style Sheet) folder contains the stylesheet for your EPUB. The stylesheet contains directions as to how different parts of your text should be formatted – what fonts to use, what size the text should be for headings and body content, and how footnotes, end notes or other material should be formatted. Rather than having to format each individual element on each XHTML page in your ebook, you can set rules using CSS to tell the reading system that headers should always be a certain size, and body paragraphs should be indented or formatted in a certain way.
Fonts
The fonts folder contains the actual font files that are used in your ebook. Typically, for each font, there will be several files, because there will be regular, bold, and italic versions of each font. If your ebook uses more than one font, you would group all of the font files within the same folder.
Images
The images folder contains any images that are displayed in the ebook. Depending on the book, there may only be one image, the cover, but it’s best practice to create an images folder even if you only have one image in the book.
Content Files
Each section of your book should have its own XHTML page. XHTML is a blend of XML and HTML, and it is the language used to format the actual content in your book, plus the copyright page, introduction, table of contents, and any other front or end matter.
The most logical way to divide the content of your book into XHTML files is to create a different file for each chapter, but if your book doesn’t have chapters, or has a lot of short chapters, you may choose to break the content up differently. It won’t make a difference to the way that the book is displayed to the reader how you divide the content among XHTML files, as long as they are all in order in your OPF file.
The OPS folder also contains the OPF file, which we have created a separate tutorial about here.