The Web Application Server consists of three layers: the database layer; the application layer; and the presentation layer. These layers may run on the same or on different physical machines. The "database layer" contains the relational database and the database software. The "application layer" knowledge contains the instance or instances of the system. All application processes, including the business transactions and the ABAP development, run on the application layer. The "presentation layer" handles the interaction with users of the system. Online access to ABAP application servers can go via a proprietary graphical interface, which is called "SAP GUI", or via a Web browser.