You might've read about layout inheritance (things not defined in a layout inherit from the core, for example) and read about the more familiar object inheritance (an undefined method Poodle::bark() inherits from Dog::bark()) and asked yourself… "How do object inheritance and layer inheritance interact? Which comes first?" Object inheritance happens first and is saved into the layer. At the very end, when layers are combined to make a vtable for a style, that's when layer inheritance happens. If none of this makes sense, don't worry about it.