access control list X maintains lists of hosts that are allowed access to each server controlling a display. By default, only the local host may use the display, plus any hosts specified in the access control list for that display. The list is found in /etc/Xn.hosts where n is the number of the display. The access control list is also known as the host access list.
active window The window where the input is directed. With a “pointer focus” window manager such as twm, you must put the pointer in a window to make it the active window. The active window is sometimes called the focus window.
ASCII American Standard Code for Information Interchange. This standard for data transmission assigns individual 7-bit codes to represent each of a specific set of 128 numerals, letters, and control characters.
background Windows may have a background, consisting of either a solid color or a tile pattern. If a window has a background, it will be repainted automatically by the server whenever there is an Expose event on the window. If a window does not have a background, it will be transparent See also foreground.
background color The color that determines the backdrop of a window, for example, on monochrome displays, the root window background color is gray.
background window A shaded area (also called the root window) that covers the entire screen and upon which other windows are displayed.
binding An association between a function and a key and/or pointer button. twm allows you to bind its functions to any key(s) on the keyboard, or to a combination of keys and pointer button (e.g., the Control key and the middle button on a 3-button pointer).
bitmap A grid of pixels or picture elements, each of which is white, black, or, in the case of color displays, a color. The bitmap client allows you to edit bitmaps, which you can use as pointers, icons, and background window patterns.
border A window can have a border that is zero or more pixels wide. If a window has a border, the border can have a solid color or a tile pattern, and it will be repainted automatically by the server whenever its color or pattern is changed or an Expose event occurs on the window.
client An X application program. There are client programs to perform a variety of tasks, including terminal emulation and window management. Clients need not run on the same system as the display server program.
colorcell An entry in a colormap is known as a colorcell. An entry contains three values specifying red, green, and blue intensities. These values are always 16-bit unsigned numbers, with zero being minimum intensity. The values are truncated or scaled by the server to match the display hardware. See also colormap.
colormap A colormap consists of a set of colorcells. A pixel value indexes into the colormap to produce intensities of red, green, and blue to be displayed. Depending on hardware limitations, one or more color- maps may be installed at one time, such that windows associated with those maps display with true colors. Regardless of the number of installable colormaps, any number of virtual colormaps can be created. When needed, a virtual colormap can be installed and the existing installed colormap may have to be uninstalled. The colormap on most systems is a limited resource that should be conserved by allocating read-only colorcells whenever possible, and selecting RGB values from the predefined color database. Read-only cells may be shared between clients. See also RGB.
console xterm window This xterm window is the first window to appear on your display. Exiting the console window kills the X server program and any associated applications. Also called the login xterm window.
default A function-dependent value assigned when you do not specify a value. For example, specifying the -rv option with xterm reverses the foreground and background colors for the xterm window. If you do not specify this option, the default foreground and background colors are used.
depth The depth of a window or pixmap is the number of bits per pixel.
device-dependent Aspects of a system that vary depending on the hardware. For example, the number of colors available on the screen (or whether color is available at all) is a device-dependent feature of X.
display A set of one or more screens driven by a single X server. The DISPLAY environment variable tells programs which servers to connect to, unless it is overridden by the -display command line option. The default is always screen 0 of (display) server 0 on the local node.
event Something that must happen before an action can occur.
exposure Window exposure occurs when a window is first mapped, or when another window that obscures it is unmapped, resized, or moved. Servers do not guarantee to preserve the contents of windows when windows are obscured or reconfigured. Expose events are sent to clients to inform them when contents of regions of windows have been lost and need to be regenerated.
focus window The window to which keyboard input is directed. By default, the keyboard focus belongs to the root, which has the effect of sending input to whichever window has the pointer in it (if you are using a “pointer focus” window manager, such as twm). However, some clients may automatically take the focus, which means they may send input to a particular window regardless of the position of the pointer.
font A style of text characters. Fonts and X font naming conventions are described in Chapter 5, Font Specification. Samples of Release 3 and 4 screen fonts are pictured in Appendix E.
font directory By default, Release 3 and Release 4 fonts are stored in three subdirectories of /usr/lib/Xll/fonts: called misc, 75dpi, and 100dpi. (Release 2 fonts are stored in the directory /usr/lib/Xll/fonts.) You can specify an alternative font search path for the server with the xset client.
foreground The pixel value that will actually be used for drawing pictures or text is referred to as the foreground.
foreground color The color in which the text in windows and menus, or graphics output are displayed.
geometry The specification for the size and placement of a window, which can be specified with the -geometry option. This option takes an argument of the form: widthxheight±xoff±yoff.
hexadecimal A base-16 arithmetic system, which uses the digits A through F to represent the base-10 numbers 10 through 15. Hexadecimal notation (called hex for short) is frequently used with computers because a single hex digit can represent four binary digits (bits). The table below shows the equivalence between hex digits and binary numbers.
X clients accept a special hexadecimal notation (prefixed by a # character) in all command line options relating to color. See Chapter 8, Command Line Options, for more information.
highlighter The horizontal band of color that moves with the pointer within a menu.
hot spot The reference point of a pointer that corresponds to its specified position on the display. In the case of an arrow, an appropriate hot spot is its tip. In the case of a cross, an appropriate hot spot might be its center.
icon A small symbol that represents a window but uses little space on the display. Converting windows to icons allows you to keep your display uncluttered.
input device Hardware device that allows you to input information to the system. For a window-based system, a keyboard and pointer are the most common input devices.
keyboard focus See focus window.
menu A list of commands or functions, listed in a small window, which can be selected with the pointer.
modifier keys Keys on the keyboard such as Control, Alt, and Shift. X programs recognize a set of “logical” modifier key functions that can be mapped to physical keys. The most frequently used of these logical keys is called the “meta” key.
mouse An input device that, when moved across a flat surface, moves the pointer symbol correspondingly across the display. The mouse usually has buttons that can be pressed to send signals that in turn accomplish certain functions. The mouse is one type of pointer device; the representation of the mouse on the screen is also called the pointer. (See pointer.)
occluding In a windowing system, windows may be stacked on top of each other much like a deck of cards. The window that overlays another window is said to occlude that window. A window need not completely conceal another window to be occluding it
padding Space inserted to maintain alignment within the borders of windows and menus.
parameter A value required before a client can perform a function. Also called an argument.
pixel The smallest element of a display surface that can be addressed.
pointer A generic name for an input device that, when moved across a flat surface, moves the pointer symbol correspondingly across the display. A pointer usually has buttons that can be pressed to send signals that in turn accomplish certain functions. A mouse is one type of pointer device.

The pointer also refers to the symbol on your display that tracks pointer movement on your desk. Pointers allow you to make selections in menus, size and position windows and icons, and select the window where you want to focus input. A pointer can be represented by a variety of symbols. (See text cursor.) Some typical X pointer symbols are the I-beam and the skull and crossbones.
property Windows have associated properties, each consisting of a name, a type, a data format, and some data. The X protocol places no interpretation on properties; they are intended as a general-purpose data storage and intercommunication mechanism for clients. There is, however, a list of predefined properties and property types so that clients can share information such as resize hints, program names, and icon formats with a window manager. In order to avoid passing arbitrary length property-name strings, each property name is associated with a corresponding integer value known as an atom.
reverse video Reversing the default foreground and background colors.
RGB An additive method for defining color in which tenths of percentages of the primaries red, green, and blue are combined to form other colors.
root window A shaded area (also called the background window) that covers the entire screen and upon which other windows are displayed.
screen A server may provide several independent screens, which may or may not have physically independent monitors. For instance, it is sometimes possible to treat a color monitor as if it were two screens, one color and one black and white.
scrollbar A bar on the side of an xterm window that allows you to use the pointer to scroll up and down through the text saved in the window. The number of lines saved is usually greater than the number of lines displayed and can be controlled by the saveLines resource variable.
select A process in which you move the pointer to the desired menu item or window and click or hold down a pointer button in order to perform some action.
selection Selections are a means of communication between clients using properties and events. From the user’s perspective, a selection is an item of data that can be highlighted in one instance of an application and pasted into another instance of the same or a different application. The client that highlights the data is the owner, and the client into which the data is pasted is the requestor. Properties are used to store the selection data and the type of the data, while events are used to synchronize the transaction and to allow the requestor to indicate the type of data it prefers and to allow the owner to convert the data to the indicated type if possible.
server The combination of graphics display, hardware, and X server software that provides display services for clients. The server also handles keyboard and pointer input.
text cursor The standard underscore or block cursor that appears on the command line or in a text editor running an xterm window. To make the distinction clearer, the cursor that tracks the movement of a mouse or other pointing device is referred to as the pointer. The pointer may be associated with any number of cursor shapes, and may change shape as it moves from window to window.
tile A pattern that is replicated (as if laying a tile) to form the background of a window or other area. This term is also used to refer to a style of window manager or application that places windows side by side instead of allowing them to overlap.
window A region on your display created by a client. For example, the xterm terminal emulator, the xcalc calculator, and the bitmap graphics editor all create windows. You can manipulate windows on your display using a window manager.
window manager A client that allows you to move, resize, circulate, and iconify windows on your display.

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