twbs--bootstrap/scss/_tooltip.scss

// Base class
.tooltip {
  position: absolute;
  z-index: $zindex-tooltip;
  display: block;
  // Our parent element can be arbitrary since tooltips are by default inserted as a sibling of their target element.
  // So reset our font and text properties to avoid inheriting weird values.
  @include reset-text();
  font-size: $font-size-sm;
  // Allow breaking very long words so they don't overflow the tooltip's bounds
  word-wrap: break-word;
  opacity: 0;

  &.active { opacity: $tooltip-opacity; }

  &.tooltip-top,
  &.bs-tether-element-attached-bottom {
    padding: $tooltip-arrow-width 0;
    margin-top: -$tooltip-margin;

    .tooltip-inner::before {
      bottom: 0;
      left: 50%;
      margin-left: -$tooltip-arrow-width;
      content: "";
      border-width: $tooltip-arrow-width $tooltip-arrow-width 0;
      border-top-color: $tooltip-arrow-color;
    }
  }
  &.tooltip-right,
  &.bs-tether-element-attached-left {
    padding: 0 $tooltip-arrow-width;
    margin-left: $tooltip-margin;

    .tooltip-inner::before {
      top: 50%;
      left: 0;
      margin-top: -$tooltip-arrow-width;
      content: "";
      border-width: $tooltip-arrow-width $tooltip-arrow-width $tooltip-arrow-width 0;
      border-right-color: $tooltip-arrow-color;
    }
  }
  &.tooltip-bottom,
  &.bs-tether-element-attached-top {
    padding: $tooltip-arrow-width 0;
    margin-top: $tooltip-margin;

    .tooltip-inner::before {
      top: 0;
      left: 50%;
      margin-left: -$tooltip-arrow-width;
      content: "";
      border-width: 0 $tooltip-arrow-width $tooltip-arrow-width;
      border-bottom-color: $tooltip-arrow-color;
    }
  }
  &.tooltip-left,
  &.bs-tether-element-attached-right {
    padding: 0 $tooltip-arrow-width;
    margin-left: -$tooltip-margin;

    .tooltip-inner::before {
      top: 50%;
      right: 0;
      margin-top: -$tooltip-arrow-width;
      content: "";
      border-width: $tooltip-arrow-width 0 $tooltip-arrow-width $tooltip-arrow-width;
      border-left-color: $tooltip-arrow-color;
    }
  }
}

// Wrapper for the tooltip content
.tooltip-inner {
  max-width: $tooltip-max-width;
  padding: $tooltip-padding-y $tooltip-padding-x;
  color: $tooltip-color;
  text-align: center;
  background-color: $tooltip-bg;
  @include border-radius($border-radius);

  &::before {
    position: absolute;
    width: 0;
    height: 0;
    border-color: transparent;
    border-style: solid;
  }
}