A sample of the spine element in an OPF file.

spine_in_opf_sample.opf

<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" dir="ltr" prefix="se: http://standardebooks.org/vocab/1.0" unique-identifier="uid" version="3.0" xml:lang="en-US">
    <spine toc="ncx">
        <itemref idref="titlepage.xhtml"/>
        <itemref idref="chapter-1.xhtml"/>
        <itemref idref="chapter-2.xhtml"/>
        <itemref idref="chapter-3.xhtml"/>
        <itemref idref="uncopyright.xhtml"/>
        <!--Other itemrefs-->
    </spine>
    <!--Other elements-->
</package>