Bootstrap/scss/utilities/_flex.scss

// Flex variation
//
// Custom styles for additional flex alignment options.

@if $enable-flex {
  @each $breakpoint in map-keys($grid-breakpoints) {
    @include media-breakpoint-up($breakpoint) {
      $min: breakpoint-min($breakpoint, $grid-breakpoints);

      @if $min {
        // Flex column reordering
        .flex-#{$breakpoint}-first     { order: -1; }
        .flex-#{$breakpoint}-last      { order: 1; }
        .flex-#{$breakpoint}-unordered { order: 0; }

        // Alignment for every item
        .flex-items-#{$breakpoint}-top    { align-items: flex-start; }
        .flex-items-#{$breakpoint}-middle { align-items: center; }
        .flex-items-#{$breakpoint}-bottom { align-items: flex-end; }

        // Alignment per item
        .flex-#{$breakpoint}-top    { align-self: flex-start; }
        .flex-#{$breakpoint}-middle { align-self: center; }
        .flex-#{$breakpoint}-bottom { align-self: flex-end; }

        // Horizontal alignment of item
        .flex-items-#{$breakpoint}-left    { justify-content: flex-start; }
        .flex-items-#{$breakpoint}-center  { justify-content: center; }
        .flex-items-#{$breakpoint}-right   { justify-content: flex-end; }
        .flex-items-#{$breakpoint}-around  { justify-content: space-around; }
        .flex-items-#{$breakpoint}-between { justify-content: space-between; }
      } @else {
        // Flex column reordering
        .flex-first     { order: -1; }
        .flex-last      { order: 1; }
        .flex-unordered { order: 0; }

        // Alignment for every item
        .flex-items-top    { align-items: flex-start; }
        .flex-items-middle { align-items: center; }
        .flex-items-bottom { align-items: flex-end; }

        // Alignment per item
        .flex-top    { align-self: flex-start; }
        .flex-middle { align-self: center; }
        .flex-bottom { align-self: flex-end; }

        // Horizontal alignment of item
        .flex-items-left    { justify-content: flex-start; }
        .flex-items-center  { justify-content: center; }
        .flex-items-right   { justify-content: flex-end; }
        .flex-items-around  { justify-content: space-around; }
        .flex-items-between { justify-content: space-between; }
      }
    }
  }
}