Hide blocks in Susy

Question

Looking at the susy site, how does it hide .secondary when getting at a breakpoint?

Answer 1

Taken from the GutHub source for the site (with other styles removed):

.guides, .tutorial {   
  .secondary { display: none; } // secondary starts hidden

  @include at-breakpoint($break) {    
    .secondary { display: block; } // secondary becomes visible at breakpoint
  }
}