A lot of applications even today are build using the layered architecture pattern. Inside a layered architecture, an application is organized into a set of horizontal layers where each layer performs a specific role.
A typical example is a 4 layer architecture with presentation, business, persistence and database layer:
In a layered architecture each layer has its own responsibilities which allows you to have a clean separation of concerns. Each layer can only talk to the layer directly below it(see the direction of the arrows). As a consequence your presentation layer cannot directly talk to the database layer directly but have to pass through all layers to finally reach the database.
By default all layers are closed. A closed layer means that as a request moves from layer to layer, it MUST go through the layer right below it to get to the layer below that one.
What some people seem to forget is that inside a layered architecture it is also possible to mark layers as open. There can be good reasons why in some situations a certain layer can be skipped. A good example is in a CQRS architecture where on the write side you have to go through the business and persistence layer to get to the database where on the read side it is ok to skip the business layer and talk to the persistence layer directly:
