position

значения свойства:

static
absolute
fixed
relative
inherit

css/style.css

* {
margin: 0;
padding: 0;
}
.a {
height: 400px;
width: 400px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
border: 1px solid #808080;
}
#div1, .b, .c {
width: 100px;
height: 100px;
background-color: #7092BE;
}
#div1 {
top: 150px;
left: 150px;
}
.c {
bottom: 25px;
left: 25px;
position: fixed;
}

* {
 margin: 0;
 padding: 0;
}

.a {
 height: 400px;
 width: 400px;
 position: absolute;
 top: 50%;
 left: 50%;
 margin-top: -200px;
 margin-left: -200px;
 border: 1px solid #808080;
}

#div1, .b, .c {
 width: 100px;
 height: 100px;
 background-color: #7092BE;
}

#div1 {
 top: 150px;
 left: 150px;
}

.c {
 bottom: 25px;
 left: 25px;
 position: fixed;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="a">
<div id="div1"></div>
</div>
<div class="b"></div>
<div class="c"></div>
<script>
/*JavaScript*/
document.getElementById("div1").style.position = "relative";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "position": "static" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="a">
<div id="div1"></div>
</div>
<div class="b"></div>
<div class="c"></div>
<script>
/*JavaScript*/
document.getElementById("div1").style.position = "relative";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "position": "static" });
});
</script>
</body>
</html>