z-index на позиции абсолютно
#relative {
position: relative;
width: 40px;
height: 100px;
background: #000;
z-index: 1;
margin-top: 30px;
}
.absolute {
position: absolute;
top: 0; left: 0;
width: 200px;
height: 50px;
background: yellow;
}
Odd Ocelot