Consider:
<!DOCTYPE html>
<html>
<head>
<style type="text/css">
.left, .right {
display: inline-block;
width: 49%;
}
.left {
height: 20px;
background-color: #dddddd;
}
.right {
height: 100px;
background-color: #eeeeee;
}
</style>
</head>
<body>
<div>
<div class="left"></div>
<div class="right"></div>
</div>
</body>
</html>
Because inline elements are vertically aligned to the baseline by default. You need to change that.