Check out example codes for "sass mixin breakpoint". It will help you in understanding the concepts better.

Code Example 1

@mixin breakpoint($class) {  
  @if $class == xs {    
	@media (max-width: 767px) { @content; }  }   
  @else if $class == sm {    
    @media (min-width: 768px) { @content; }  }   
  @else if $class == md {    
    @media (min-width: 992px) { @content; }  }   
  @else if $class == lg {    
    @media (min-width: 1200px) { @content; }  }   
  @else {    
    @warn "Breakpoint mixin supports: xs, sm, md, lg";  
  }
}

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