Bootstrap/scss/utilities/_flex.scss

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

@if $enable-flex {
  @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 item
    @include media-breakpoint-up($breakpoint) {
      .flex-items-#{$breakpoint}-top { align-items: flex-start; }
      .flex-items-#{$breakpoint}-middle { align-items: center; }
      .flex-items-#{$breakpoint}-bottom { align-items: flex-end; }
    }

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

    // Horizontal alignment of item
    @include media-breakpoint-up($breakpoint) {
      .flex-items-#{$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; }
    }
  }
}