// breakpoints
   
$S:     320px;   
$M:     768px;     
$L:     1170px;     

// media queries

@mixin MQ($canvas) {
  @if $canvas == S {
   @media only screen and (min-width: $S) { @content; } 
  }
  @else if $canvas == M {
   @media only screen and (min-width: $M) { @content; } 
  }
  @else if $canvas == L {
   @media only screen and (min-width: $L) { @content; } 
  }
}