With having the big document broken up into many little pages, it's necessary to provide a common method to skip through all these pages, going back and forth. Also, when reading some page, it might be useful to be able to go to the index and/or table of contents as well. The mechanism providing this is called a ``navigation bar''. In PDF, this is built into the document format and the reader itself, so nothing has to be done there, but in HTML this has to be realized by hand. The navigation bar provides a doubly linked list of all the pages, for going to the next and previous page, see Figure 9.
The buttons a navigation bar should provide at the very least links to the next and last page. The ``back''-link from the table of contents and the ``next''-link from the last page are usually omitted. Additional links to the table of contents and index make it much more usable.
Also, a ``up'' link may provide a way to go up one step in the document hierarchy, as is shown by Figure 10.
The ``up''-links only go one level up to the next hightest page, as for example the ``up''-link for 1.2.1 goes to 1.2.
Aside from the logical structure of the links, the actual look of these links in the navigation bar may vary between documents (although consistency is a must within a single document!). The links may consist of pictures, chapter numbers, chapter names, or both, see Figures 11, 12, 13 and 14 for examples.
When placing navigations bars, attention should be paid that there is one at the beginning of the page, and also a second one at the end, so the reader of the page doesn't have to scroll up when he's done reading the page, but instead can go to the next page directly.