HTML

bootstrap table combine columns

<thead>
  <tr>
    <th rowspan="2">State</th>
    <th rowspan="2">Utilities Company</th>
    <th colspan="3">Summer Period</th>
  </tr>
  <tr>
    <th>data1</th>
    <th>data2</th>
    <th>data3</th>
  </tr>
</thead>