5.2 Understanding X

5.2.1 Why X?

X is not the first window system written for UNIX®, but it is the most popular. X's original development team had worked on another window system before writing X. That system's name was ``W'' (for ``Window''). X is just the next letter in the Roman alphabet.

X can be called ``X'', ``X Window System'', ``X11'', and other terms. Calling X11 ``X Windows'' can offend some people; see X(7) for a bit more insight on this.

5.2.2 The X Client/Server Model

X was designed from the beginning to be network-centric, and adopts a ``client-server'' model. In the X model, the ``X server'' runs on the computer that has the keyboard, monitor, and mouse attached. The server is responsible for managing the display, handling input from the keyboard and mouse, and so on. Each X application (such as XTerm, or Netscape®) is a ``client''. A client sends messages to the server such as ``Please draw a window at these coordinates'', and the server sends back messages such as ``The user just clicked on the OK button''.

If there is only one computer involved, such as in a home or small office environment, the X server and the X clients will be running on the same computer. However, it is perfectly possible to run the X server on a less powerful desktop computer, and run X applications (the clients) on, say, the powerful and expensive machine that serves the office. In this scenario the communication between the X client and server takes place over the network.

This confuses some people, because the X terminology is exactly backward to what they expect. They expect the ``X server'' to be the big powerful machine down the hall, and the ``X client'' to be the machine on their desk.

Remember that the X server is the machine with the monitor and keyboard, and the X clients are the programs that display the windows.

There is nothing in the protocol that forces the client and server machines to be running the same operating system, or even to be running on the same type of computer. It is certainly possible to run an X server on Microsoft Windows or Apple's Mac OS, and there are various free and commercial applications available that do exactly that.

The X server that ships with FreeBSD is called XFree86™, and is available for free, under a license very similar to the FreeBSD license. Commercial X servers for FreeBSD are also available.

5.2.3 The Window Manager

The X design philosophy is much like the UNIX design philosophy, ``tools, not policy''. This means that X does not try to dictate how a task is to be accomplished. Instead, tools are provided to the user, and it is the user's responsibility to decide how to use those tools.

This philosophy extends to X not dictating what windows should look like on screen, how to move them around with the mouse, what keystrokes should be used to move between windows (i.e., Alt+Tab, in the case of Microsoft Windows), what the title bars on each window should look like, whether or not they have close buttons on them, and so on.

Instead, X delegates this responsibility to an application called a ``Window Manager''. There are dozens of window managers available for X: AfterStep, Blackbox, ctwm, Enlightenment, fvwm, Sawfish, twm, Window Maker, and more. Each of these window managers provides a different look and feel; some of them support ``virtual desktops''; some of them allow customized keystrokes to manage the desktop; some have a ``Start'' button or similar device; some are ``themeable'', allowing a complete change of look-and-feel by applying a new theme. These window managers, and many more, are available in the x11-wm category of the Ports Collection.

In addition, the KDE and GNOME desktop environments both have their own window managers which integrate with the desktop.

Each window manager also has a different configuration mechanism; some expect configuration file written by hand, others feature GUI tools for most of the configuration tasks; at least one (sawfish) has a configuration file written in a dialect of the Lisp language.

5.2.4 Widgets

The X approach of providing tools and not policy extends to the widgets that seen on screen in each application.

``Widget'' is a term for all the items in the user interface that can be clicked or manipulated in some way; buttons, check boxes, radio buttons, icons, lists, and so on. Microsoft Windows calls these ``controls''.

Microsoft Windows and Apple's Mac OS both have a very rigid widget policy. Application developers are supposed to ensure that their applications share a common look and feel. With X, it was not considered sensible to mandate a particular graphical style, or set of widgets to adhere to.

As a result, do not expect X applications to have a common look and feel. There are several popular widget sets and variations, including the original Athena widget set from MIT, Motif® (on which the widget set in Microsoft Windows was modeled, all bevelled edges and three shades of grey), OpenLook, and others.

Most newer X applications today will use a modern-looking widget set, either Qt, used by KDE, or GTK, used by the GNOME project. In this respect, there is some convergence in look-and-feel of the UNIX desktop, which certainly makes things easier for the novice user.