Modal

<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
  <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
  <button type="button" class="close" data-dismiss="modal" aria-label="Close">
    <span aria-hidden="true">&times;</span>
  </button>
</div>
<div class="modal-body">
  ...
</div>
<div class="modal-footer">
  <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
  <button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>

<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
<div class="modal-dialog modal-dialog-centered" role="document">
<div class="modal-content">
<div class="modal-header">
  <h5 class="modal-title" id="exampleModalLongTitle">Modal title</h5>
  <button type="button" class="close" data-dismiss="modal" aria-label="Close">
    <span aria-hidden="true">&times;</span>
  </button>
</div>
<div class="modal-body">
  ...
</div>
<div class="modal-footer">
  <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
  <button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>