Generated by SassMeister.com.

SassMeister-input-HTML.html

<h3>Mixin example for <a href="http://blog.teamtreehouse.com/sass-tip-double-ampersand-selector">this blog post</a>.</h3>
<div class="btn-group">
  <a class="btn" href="#">Primary Action</a>
  <a class="btn" href="#">Secondary Action</a>
</div>
<div class="row">
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.0.rc.1)
// ----

// Boring stuff
body {font-family: sans-serif; line-height: 1.5;}
.btn-group {margin: 30px 0;}

// Double ampersand selector mixin
@mixin doubly($margin: 1.1em) {
	& + & {
	  margin-left: $margin;
	  @content;
	}
}

// Using them

.btn {
  display: inline-block;
  text-decoration: none;
  background: dodgerblue;
  color: white;
  padding: 10px 20px;
  border-radius: .5em;
  @include doubly {
    background: tomato;
  }
}

.col {
  width: 30%;
  float: left;
  @include doubly(5%);
}

SassMeister-output.css

body {
  font-family: sans-serif;
  line-height: 1.5;
}

.btn-group {
  margin: 30px 0;
}

.btn {
  display: inline-block;
  text-decoration: none;
  background: dodgerblue;
  color: white;
  padding: 10px 20px;
  border-radius: .5em;
}
.btn + .btn {
  margin-left: 1.1em;
  background: tomato;
}

.col {
  width: 30%;
  float: left;
}
.col + .col {
  margin-left: 5%;
}

SassMeister-rendered.html

<h3>Mixin example for <a href="http://blog.teamtreehouse.com/sass-tip-double-ampersand-selector">this blog post</a>.</h3>
<div class="btn-group">
  <a class="btn" href="#">Primary Action</a>
  <a class="btn" href="#">Secondary Action</a>
</div>
<div class="row">
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
  <div class="col">
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam suscipit in tortor non laoreet. In porttitor elit non tellus ultrices, eget mattis risus ornare. Nunc commodo consectetur fermentum. 
  </div>
</div>