Problem with z-index

[sciencebear]

Hey everyone. I'm doing a mockup of a new layout, and I can't seem to get the z-index right. You can see the site here: NEVER MIND-just needed to add position:relative;

 

Basically, the div called "atab" should have a white background that I want to cover up part of the left border on the "main" div. I'm pretty sure I set the z-indexs right, but it still doesn't seem to work. Any help?

 

This is my CSS [though you should be able to see in in the source of the site]

 

body {
background-image: url(images/bg.gif);
}

.shell {
width:890px;
text-align:center;
margin-right:auto;
margin-left:auto;
}

.navtabs {
background:#FFFFFF;
width:25px;
float:left;
border-top:1px solid #000000;
border-left:1px solid #000000;
border-bottom:1px solid #000000;
z-index:1;
}

.main {
background:#FFFFFF;
width:862px;
float:right;
border:1px solid #000000;
min-height:500px;
z-index:0;
}

.itab {
background:#FFFFFF;
height:42px;
width:25px;
border-bottom:1px solid #000000;
}

.atab {
background:#FFFFFF;
height:42px;
width:26px;
border-bottom:1px solid #000000;
z-index:2;
}

.btab {
background:#FFFFFF;
width:25px;
height:42px;
}