vertical-align

* {
 padding: 0;
 margin: 0;
}

body {
 background-color: #FFF5EE;
}

.main {
 width: 900px;
 height: 500px;
 position: absolute;
 left: 50%;
 top: 50%;
 margin-left: -450px;
 margin-top: -250px;
}

.left, .right {
 width: 270px;
 height: 500px;
 float: left;
 border: 1px solid #478E8E;
 background-color: #FFF;
}

.center {
 width: 270px;
 height: 500px;
 float: left;
 border: 1px solid #478E8E;
 background-color: #FFF;
 display: table;
}

.empty {
 width: 42px;
 height: 500px;
 float: left;
}

.a {
 vertical-align: top;
}

.c {
 vertical-align: top;
 display: table-cell;
}

.d {
 vertical-align: middle;
 display: table-cell;
}

.e {
 vertical-align: bottom;
 display: table-cell;
}

.f {
 vertical-align: super;
 background-color: #FFD700;
}

.g {
 vertical-align: sub;
 background-color: #FFD700;
}

.h {
 vertical-align: 100px;
 background-color: #9ACD32;
}

.k {
 vertical-align: -100px;
 background-color: #9ACD32;
}