The h1, h2, h3, h4, h5, and h6 tags are used to specify headings in a document. The h1 tag produces the largest heading, while each successive number is slightly smaller than the previous. Here is a table showing the default heading sizes:

HTML Output
<h1>Hello World</h1> Hello World
<h2>Hello World</h2> Hello World
<h3>Hello World</h3> Hello World
<h4>Hello World</h4> Hello World
<h5>Hello World</h5> Hello World
<h6>Hello World</h6> Hello World