The <rel> and <rev> attributes define the relationship between the source and destination documents

There are links from Contents page of My Book to each Chapter and from each Chapter back to Contents page of My Book but there is a single relationship between Contents page of My Book and its Chapters

rel

Describes the relationship from the current document to the destination document

rev

Describes a reverse link. Gives the relationship from the destination document to the current document

Contents.htm

<a href="Chapter1.htm" rel="Chapter">Chapter 1</a>

Chapter1.htm

<a href="Contents.htm" rel="Contents" rev="Chapter" >Back 
   to Contents Page</a>