twbs--bootstrap/scss/forms/_labels.scss

//
// Labels
//

// For use with horizontal and inline forms, when you need the label (or legend)
// text to align with the form controls.
.col-form-label {
  padding-top: calc(#{$input-padding-y} + #{$input-border-width});
  padding-bottom: calc(#{$input-padding-y} + #{$input-border-width});
  margin-bottom: 0; // Override the `<label>/<legend>` default
  @include font-size(inherit); // Override the `<legend>` default
  line-height: $input-line-height;
}

.col-form-label-lg {
  padding-top: calc(#{$input-padding-y-lg} + #{$input-border-width});
  padding-bottom: calc(#{$input-padding-y-lg} + #{$input-border-width});
  @include font-size($input-font-size-lg);
}

.col-form-label-sm {
  padding-top: calc(#{$input-padding-y-sm} + #{$input-border-width});
  padding-bottom: calc(#{$input-padding-y-sm} + #{$input-border-width});
  @include font-size($input-font-size-sm);
}