Check out example codes for "scss extend". It will help you in understanding the concepts better.

Code Example 1

.myclass {
  font-weight: bold;
  font-size: 90px;
}

.myotherclass {
  @extend .myclass;
  color: #000000;
}

Code Example 2

.error:hover {
  background-color: #fee;
}

.error--serious {
  @extend .error;
  border-width: 3px;
}

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