Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

.cat {
  color: gold;
    
  // must be both a cat and black in the same class=".."
  &.black {
  	color: black;
  }
  
  // you can have a div with class black INSIDE of a div with class cat
  .black {
  	color: black
  }
}

SassMeister-output.css

.cat {
  color: gold;
}
.cat.black {
  color: black;
}
.cat .black {
  color: black;
}