Magical mixins with trees

sass-mixins.scss

@mixin gallery-items($size) {
.item {
    float: left;
    .thumbnail {
      margin-bottom: $size/4;

      text-align: center;
      width: $size;
      height: $size;

      .vertical-helper {
        display: inline-block;
        height: 100%;
        vertical-align: middle;
        width: 1px;
      }
    }
  }
}

#some-page {
  .gallery {
    @include gallery-items(200px);
  }
}

#some-other-page {
  #other-element {
    .another-gallery {
      @include gallery-items(400px);
    }
  }
}