Bootstap Media Breakpoints:

@include media-breakpoint-up(xs) { ... }
@include media-breakpoint-up(sm) { ... }
@include media-breakpoint-up(md) { ... }
@include media-breakpoint-up(lg) { ... }
@include media-breakpoint-up(xl) { ... }

// Example usage:
@include media-breakpoint-up(sm) {
  .some-class {
    display: block;
  }
}

@include media-breakpoint-down(xs) { ... }
@include media-breakpoint-down(sm) { ... }
@include media-breakpoint-down(md) { ... }
@include media-breakpoint-down(lg) { ... }

@include media-breakpoint-only(xs) { ... }
@include media-breakpoint-only(sm) { ... }
@include media-breakpoint-only(md) { ... }
@include media-breakpoint-only(lg) { ... }
@include media-breakpoint-only(xl) { ... }



GRID

$grid-columns:      12;
$grid-gutter-width-base: 30px;

$grid-gutter-widths: (
  xs: $grid-gutter-width-base, // 30px
  sm: $grid-gutter-width-base, // 30px
  md: $grid-gutter-width-base, // 30px
  lg: $grid-gutter-width-base, // 30px
  xl: $grid-gutter-width-base  // 30px
)

$grid-breakpoints: (
  // Extra small screen / phone
  xs: 0,
  // Small screen / phone
  sm: 576px,
  // Medium screen / tablet
  md: 768px,
  // Large screen / desktop
  lg: 992px,
  // Extra large screen / wide desktop
  xl: 1200px
);

$container-max-widths: (
  sm: 540px,
  md: 720px,
  lg: 960px,
  xl: 1140px
);

// Creates a wrapper for a series of columns
@mixin make-row($gutters: $grid-gutter-widths) {
  display: flex;
  flex-wrap: wrap;

  @each $breakpoint in map-keys($gutters) {
    @include media-breakpoint-up($breakpoint) {
      $gutter: map-get($gutters, $breakpoint);
      margin-right: ($gutter / -2);
      margin-left:  ($gutter / -2);
    }
  }
}

// Make the element grid-ready (applying everything but the width)
@mixin make-col-ready($gutters: $grid-gutter-widths) {
  position: relative;
  // Prevent columns from becoming too narrow when at smaller grid tiers by
  // always setting `width: 100%;`. This works because we use `flex` values
  // later on to override this initial width.
  width: 100%;
  min-height: 1px; // Prevent collapsing

  @each $breakpoint in map-keys($gutters) {
    @include media-breakpoint-up($breakpoint) {
      $gutter: map-get($gutters, $breakpoint);
      padding-right: ($gutter / 2);
      padding-left:  ($gutter / 2);
    }
  }
}

@mixin make-col($size, $columns: $grid-columns) {
  flex: 0 0 percentage($size / $columns);
  width: percentage($size / $columns);
  // Add a `max-width` to ensure content within each column does not blow out
  // the width of the column. Applies to IE10+ and Firefox. Chrome and Safari
  // do not appear to require this.
  max-width: percentage($size / $columns);
}

// Get fancy by offsetting, or changing the sort order
@mixin make-col-offset($size, $columns: $grid-columns) {
  margin-left: percentage($size / $columns);
}

@mixin make-col-push($size, $columns: $grid-columns) {
  left: if($size > 0, percentage($size / $columns), auto);
}

@mixin make-col-pull($size, $columns: $grid-columns) {
  right: if($size > 0, percentage($size / $columns), auto);
}

.container {
  max-width: 60em;
  @include make-container();
}
.row {
  @include make-row();
}
.content-main {
  @include make-col-ready();

  @media (max-width: 32em) {
    @include make-col(6);
  }
  @media (min-width: 32.1em) {
    @include make-col(8);
  }
}
.content-secondary {
  @include make-col-ready();

  @media (max-width: 32em) {
    @include make-col(6);
  }
  @media (min-width: 32.1em) {
    @include make-col(4);
  }
}

// NAVBAR

<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
  <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <a class="navbar-brand" href="#">Navbar</a>

  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
      <li class="nav-item">
        <a class="nav-link disabled" href="#">Disabled</a>
      </li>
    </ul>
    <form class="form-inline my-2 my-lg-0">
      <input class="form-control mr-sm-2" type="text" placeholder="Search">
      <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
    </form>
  </div>
</nav>