SOURCE

<div id="box">
  <span class="cbox">
    xg
  </span>
  <span class="line"></span>
</div>

div {
  width: 100px;
  height: 100px;
  display: inline-block;
  border: 1px solid #f00;
}
span {
  display: inline-block;
  border: 1px solid #000;
  width: 40px;
}
#box {
  position: absolute;
}
.cbox {
  height: 30px;
  font-size: 20px;
}
.line {
  display: block;
  height: 1px;
  border: 0;
  background: green;
  position: absolute;
  top: 21px;
}