Generated by SassMeister.com.

SassMeister-input-HTML.html

<ul class="nav--primary">
  <li class="nav__item">One</li>
  <li class="nav__item">Two</li>
  <li class="nav__item">Three</li>
</ul>

<ul class="nav--secondary">
  <li class="nav__item">One</li>
  <li class="nav__item">Two</li>
  <li class="nav__item">Three</li>
</ul>

SassMeister-input.scss

// ----
// Sass (v3.3.9)
// Compass (v1.0.0.alpha.20)
// ----

%nav {
  background: #ccc;
  padding: 0;
  width: 100%;
  list-style-type: none;
  
  li {
    margin: 0;
    padding: 0;
  }
}

.nav--primary,
.nav--secondary {
  @extend %nav;
}

.nav--secondary {
  width: 200px;
  
  li {
    display: inline-block;
  }
}

SassMeister-output.css

.nav--primary,
.nav--secondary {
  background: #ccc;
  padding: 0;
  width: 100%;
  list-style-type: none;
}
.nav--primary li,
.nav--secondary li {
  margin: 0;
  padding: 0;
}

.nav--secondary {
  width: 200px;
}
.nav--secondary li {
  display: inline-block;
}

SassMeister-rendered.html

<ul class="nav--primary">
  <li class="nav__item">One</li>
  <li class="nav__item">Two</li>
  <li class="nav__item">Three</li>
</ul>

<ul class="nav--secondary">
  <li class="nav__item">One</li>
  <li class="nav__item">Two</li>
  <li class="nav__item">Three</li>
</ul>