Header tags should be used sparingly to separate distinct parts of the document.

Header 1 & Header 2

The Header 1 tag <h1> should only be used once on a page for the title of the main content.  The phrase "Headers" at the top of this page is an example of the Header 1 tag.

The Header 2 tag <h2> is generally reserved for navigational elements.  The menu options at the top of the page ("Research", "Services", etc.) and the phrase "Web Design Guidelines" at the top of the table of contents on the left are examples of Header 2 tags.

Header 3 & Header 4

The Header 3 tag <h3> should be the first level used in the main content, after the <h1> tag at the top.  The phrase "Header 3 & Header 4" at the top of this section is an example of the Header 3 tag.

Header 4

The Header 4 tag <h4> should be used to further divide sections between the Header 3 tags.  Many pages may not need Header 4 tags.  The phrase "Header 4" above is an example of the Header 4 tag.

Header 5 & Header 6

Currently, Header 5 <h5> and Header 6 <h6> are not styled in the library's base theme.  If you find that you need this level of division on a single page, you should consider breaking up the content into separate pages.