For people coming to Backdrop from Drupal 7 (and those starting Backdrop from scratch), layouts are often one of the most difficult new concepts to grasp. One of the challenging aspects can be figuring out (or setting) which URL(s) a given layout will be applied on. In this blog post, I’ll discuss some of the technical aspects of layouts that might help users build an overall “mental model” of how layouts work.
To start, let me point out some existing useful links relevant to layouts: