The X Window System supports one or more screens containing overlapping windows and subwindows. A screen is a physical monitor and hardware, which can be either color or black and white. There can be multiple screens per display workstation. A single server can provide display services for any number of screens. A set of screens for a single user with one keyboard and one mouse is called a display.

All windows in an X server are arranged in a strict hierarchy. At the top of the hierarchy are the root windows, which cover each of the display screens. Each root window is either partially or completely covered by child windows. All windows, except for root windows, have parents. Any window can in turn have its own children. In this way, an application program can create a window tree of arbitrary depth on each screen.

A child window can be larger than its parent. That is, part or all of the child window can extend beyond the boundaries of the parent. However, all output to a window is clipped by the boundaries of its parent window. If several children of a window have overlapping locations, one of the children is considered to be on top of/or raised over the others, obscuring them. Window output to areas that are covered by other windows is suppressed.

A window has a border that is zero or more pixels in width and can be any pattern (pixmap) or solid color. A window usually has a background pattern that is drawn by the X server. Each window has its own coordinate system. Child windows obscure their parents unless the child windows have no background. Graphics operations in the parent window are usually clipped by the children.

X also provides objects called pixmaps for off-screen storage of graphics. Single-plane pixmaps (that is, of depth 1) are sometimes referred to as bitmaps. Both pixmaps and windows can be used interchangeably in most graphics functions. Pixmaps are also used in various graphics operations to define patterns, or tiles. Windows and pixmaps together are referred to as drawables.