save set
The save set of a client is a list of other client's windows that, if they are inferiors of one of the client's windows at connection close, should not be destroyed and that should be remapped if it is unmapped. Save sets are typically used by window managers to avoid lost windows if the manager should terminate abnormally.
A list of pixel or bit values viewed as a horizontal row (all values having the same y coordinate) of an image, with the values ordered by increasing x coordinate.
scanline order
An image represented in scanline order contains scanlines ordered by increasing y coordinate.
A server can provide several independent screens, which typically have physically independent monitors. This would be the expected configuration when there is only a single keyboard and pointer shared among the screens.

A selection can be thought of as an indirect property with dynamic type. That is, rather than having the property stored in the server, it is maintained by some client (the owner). A selection is global in nature, being thought of as belonging to the user (but maintained by clients), rather than being private to a particular window subhierarchy or a particular set of clients. When a client asks for the contents of a selection, it specifies a selection target type. This target type can be used to control the transmitted representation of the contents.

For example, if the selection is "the last thing the user clicked on" and that is currently an image, then the target type might specify whether the contents of the image should be sent in XY Format or Z Format. The target type can also be used to control the class of contents transmitted; that is, asking for the looks (fonts, line spacing, indentation, and so forth) of a paragraph selection, rather than the text of the paragraph. The target type can also be used for other purposes; the semantics is not constrained by the protocol.

A condition in which a user interface component of an application will accept input. Conversely, when a contact is insensitive, events of particular types are not dispatched to the contact and are ignored.
The server provides the basic windowing mechanism. It handles IPC connections from clients, demultiplexes graphics requests onto the screens, and multiplexes input back to the appropriate clients.
server grabbing
The server can be grabbed by a single client for exclusive use. This prevents processing of any requests from other client connections until the grab is complete. This is typically only a transient state for such things as rubber-banding and pop-up menus, or to execute requests indivisibly.
server resource
Windows, pixmaps, cursors, fonts, gcontexts, and colormaps are known as resources. They all have unique identifiers associated with them for naming purposes. The lifetime of a resource is bounded by the lifetime of the connection over which the resource was created. See resource.
A composite that handles the duties required by standard conventions for top-level X windows.
Children of the same parent window are known as sibling windows.
static color
A degenerate case of pseudo color in which the RGB values are predefined and read-only.
static gray
A degenerate case of gray scale in which the gray values are predefined and read-only. The values are typically (near-)linear increasing ramps.
stacking order
Sibling windows can stack on top of each other. Windows above both obscure and occlude lower windows. This is similar to paper on a desk. The relationship between sibling windows is known as the stacking order.
A slot of contact that controls the visual effect of the contact.
A bitmap that is used to tile a region to serve as an additional clip mask for a fill operation with the foreground color.