<style>
.rel {
    position: relative;
    height: 20px;
    border: 2px solid red;
}
.abs {
    position: absolute;
    border: 2px solid green;
    width: 100px;
    display: block;
}
</style>
<div class=rel>
<div class=abs style="height:50%">
</div>
</div>
<div class=rel>
<div class=abs style="top:50%">
</div>
</div>
<div class=rel>
<div class=abs style="bottom:0px">
</div>
</div>