Bootstrap CSS class .card-subtitle

<div class="card">
  <div class="card-body">
    <h4 class="card-title">Card title</h4>
    <h6 class="card-subtitle mb-2 text-muted">Card subtitle</h6>
    <p class="card-text">
      Some quick example text to build on the card title
      and make up the bulk of the card's content.
    </p>
    <a href="#" class="card-link">Card link</a>
    <a href="#" class="card-link">Another link</a>
  </div>
</div>

/* _card.scss:45 */
.card-subtitle {
  margin-top: -($card-spacer-y / 2);
  margin-bottom: 0;
}