Open eBook (or OEB), or formally, the Open eBook Publication Structure (OEBPS), is a legacy e-book format which has been superseded by the EPUB format. It was “based primarily on technology developed by SoftBook Press” and on XML. Open eBook is a ZIP file plus a Manifest file. Inside the package a defined subset of XHTML may be used, along with CSS and Dublin Core metadata. The default file extension is . opf (OEB Package Format).