SOURCE

<div id="box">
  <span class="cbox1">xg</span>
  <span class="cbox2">xg</span>
  <span class="line"></span>
</div>

div {
  display:inline-block;
  width: 100px;
  height: 100px;
  border: 1px solid #f00;
}

span {
  width: 40px;
  border: 1px solid #000;
  display:inline-block;
}

.cbox1 {
  font-size: 20px;
  height: 30px;
}

.cbox2 {
  font-size: 12px;
  height: 20px;
}
.line {
  display: block;
  position: absolute;
  top: 31px;
  height: 1px;
  width: 100%;
  border: 0;
  background: green;
}