Cornerstone started as the built in page builder for the popular X theme, and then became an autonomous page builder plugin. One of the things we loved about Cornerstone was the intuitive interface. You really don’t need a long time to get the hang of using it. With Cornerstone, you get good control over the styling and layout, and the drag and drop feature is very smooth. All in all this is a solid page builder, even though it is not commonly used outside X theme.