Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="post">
  <div class="title">Title</div>
  <div class="alt-title">Alt title</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.8)
// Compass (v1.0.0.alpha.19)
// ----

// Shows how you can extend a class that is 
// nested without including the parent class

.title {
  border-left: 10px solid white;
  padding-left: 10px;
}

.post {
  max-width: 200px;
  border-radius: 5px;
  background: hotpink;
  padding: 15px;
  .title {
    color: white;
    font-size: 180%;
    font-family: sans-serif;
    margin: 5px 0;
    display: inline-block;
    width: 100%;
  }
  .alt-title {
    @extend .title; // Extends .title & .post .title
    font-size: 120%;
  }
}

SassMeister-output.css

.title, .post .alt-title {
  border-left: 10px solid white;
  padding-left: 10px;
}

.post {
  max-width: 200px;
  border-radius: 5px;
  background: hotpink;
  padding: 15px;
}
.post .title, .post .alt-title {
  color: white;
  font-size: 180%;
  font-family: sans-serif;
  margin: 5px 0;
  display: inline-block;
  width: 100%;
}
.post .alt-title {
  font-size: 120%;
}

SassMeister-rendered.html

<div class="post">
  <div class="title">Title</div>
  <div class="alt-title">Alt title</div>
</div>