Bootstrap/scss/utilities/_flex.scss

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

@if $enable-flex and $enable-grid-classes {
  @each $breakpoint in map-keys($grid-breakpoints) {
    // Flex column reordering
    @include media-breakpoint-up($breakpoint) {
      .flex-#{$breakpoint}-first { order: -1; }
      .flex-#{$breakpoint}-last  { order: 1; }
    }

    // Alignment for every column in row
    @include media-breakpoint-up($breakpoint) {
      .flex-items-#{$breakpoint}-top    { align-items: flex-start; }
      .flex-items-#{$breakpoint}-center { align-items: center; }
      .flex-items-#{$breakpoint}-bottom { align-items: flex-end; }
    }

    // Alignment per column
    @include media-breakpoint-up($breakpoint) {
      .flex-#{$breakpoint}-top    { align-self: flex-start; }
      .flex-#{$breakpoint}-center { align-self: center; }
      .flex-#{$breakpoint}-bottom { align-self: flex-end; }
    }
  }
}