Why won't my button go to the bottom of my header?

[DrMarie]

I want my button to be on the bottom of the header, but it stays at the top. The left margin works ok. Here is the css:

.header {
width:827px;
height:244px;
background-image:url(/images/top.jpg);
background-repeat:no-repeat;
margin-left: auto;
margin-right:auto;
margin-top:50px;
}

.buttons {
margin-bottom: 0px;
margin-left: 100px;
float:left;
background-color: #000000;
width:150px;
height:51px;
position:relative;
z-index:1;
}

and the html:

<div class="header">
<div class="buttons">
<img src="/images/pricing.gif" alt="home staging pricing" height="51" width="100">
</div>
</div>

Thanks!

[DrMarie]

Well I got it to work by setting the top margin. Not sure why it wouldn't work the way I had it though.