How to stop <div>s from moving into each other with float defined?

Very straightforward CSS question that I haven't been able to find the answer to so far:

I'm trying to lay out a page with two div's side-by-side in one row (using float:left; and float:right;) and then one div below them. The problem is that if the top row (defined as a div itself) is so wide that the space between the two divs can accomodate the bottom div, the bottom div moves up into the top row creating the appearance of a single row of three divs. I don't know if that's clear or not, but here's the code:

<div id="top div" style="width:400px;">
<div style="float:left;">开发者_高级运维<img src="images/xlab.jpg" width="100px" height="200px" /></div>
<div style="float:right;"><img src="images/ucbseal.jpg" width="100px" height="250px" /></div>
</div>

<div id="bottom div"><img src="images/xlab.jpg" width="200px" height="200px" /></div>

So, as above, since the top div has a 200px gap in between its left and right child elements, the image in the bottom div slides up between them. If I make the top div's width 399px that doesn't happen. I tried using the CSS "clear" property but that didn't solve the problem. I've always just worked my way around this seemingly-odd behavior in a sloppy way but want to find a better practice.

Any help or direction is greatly appreciated!

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Use overflow:auto on the first div

<div id="top div" style="width:400px;overflow:auto;">
<div style="float:left;"><img src="images/xlab.jpg" width="100px" height="200px" /></div>
<div style="float:right;"><img src="images/ucbseal.jpg" width="100px" height="250px" /></div>
</div>

<div id="bottom div"><img src="images/xlab.jpg" width="200px" height="200px" /></div>

it will cause the container div to expand to the contents of its children and so the following div will maintain its location..

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Try the 'clear' attribute on a new div:

<div id="top_div" style="width:400px;"> 
<div style="float:left;"><img src="images/xlab.jpg" width="100px" height="200px" /></div> 
<div style="float:right;"><img src="images/ucbseal.jpg" width="100px" height="250px" /></div> 
<div style="clear: both;" />
</div> 

<div id="bottom_div"><img src="images/xlab.jpg" width="200px" height="200px" /></div>

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I would add a div to clear the floats:

<div id="top div" style="width:400px;">
<div style="float:left;"><img src="images/xlab.jpg" width="100px" height="200px" /></div>
<div style="float:right;"><img src="images/ucbseal.jpg" width="100px" height="250px" /></div>
<div style="clear:both;"></div>
</div>

<div id="bottom div"><img src="images/xlab.jpg" width="200px" height="200px" /></div>

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I think the suggestions above concerning style="clear:both;" are spot on. However, I might try adding that style to your existing "bottom row" div. It might save you adding a new div.

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I hope its not late but I was having the same problem and just figured this out. Has to do with the bottom Div. Set height and width to 100% for the bottom div and set overflow auto on the bottom div as well. Now the div will not move up to fill space.