Sandbox v1.6 & #primary

In looking at the theme with Firebug I see there is an area with the ID ‘#primary’. I see that there is CSS for the ID primary in style.css:
div#primary {
margin-top: 2em;
}
style.css (line 59)
div.sidebar {
float: right;
overflow: hidden;
width: 180px;
}

I also see the ID ‘#primary’ is of the class sidebar. My question is where is ‘#primary’ created in index.php? I see no reference to any ID ‘#primary’ in index.php. Please help!

Thank you!