My respond-to mixin.

respond-to.scss

$small = 400px;
$medium = 800px;
$upto-medium = $medium, max-width;

@mixin respond-to($primary, $secondary:min-width) {
  @if $secondary == max-width {
    @media screen and (max-width: $primary - 1) { @content; }
  }
  @else if $secondary == min-width {
    @media screen and (min-width: $primary) { @content; }
  }
  @else {
    @media screen and (min-width: $primary) and (max-width: $secondary - 1) { @content; }
  }
}

// Usesage

@include respond-to(500px) { //min-width is default
}

@include respond-to(800px, max-width) {
}

@include respond-to(500px, 800px) {
}

@include respond-to($small) {
}

@include respond-to($upto-medium) {
}