- 
        padding 
      
- 
        Some padding bytes are inserted in the data stream to maintain
        alignment of the protocol requests on natural boundaries. This
        increases ease of portability to some machine architectures. 
      
- 
        parent window 
      
- 
        If C is a child of P, then P is the parent of C. 
      
- 
        passive grab 
      
- 
        Grabbing a key or button is a passive grab. The grab activates
        when the key or button is actually pressed. 
      
- 
        pixel value 
      
- 
        An n-bit value, where n is the number of bit planes used
        in (that is, the depth of) a particular window or pixmap. For a
        window, a pixel value indexes a colormap to derive an actual color to
        be displayed. 
      
- 
        pixmap 
      
- 
        A three dimensional array of bits. A pixmap is normally thought of as
        a two dimensional array of pixels, where each pixel can be a value
        from 0 to (2n)-1, where n is the depth (z
        axis) of the pixmap. A pixmap can also be thought of as a stack of
        n bitmaps. 
      
- 
        plane 
      
- 
        When a pixmap or window is thought of as a stack of bitmaps, each
        bitmap is called a plane or bit plane. 
      
- 
        plane mask 
      
- 
        Graphics operations can be restricted to only affect a subset of bit
        planes of a destination. A plane mask is a bit mask describing
        which planes are to be modified, and it is stored in a graphics
        context. 
      
- 
        pointer 
      
- 
        The pointing device attached to the cursor and tracked on the screens. 
      
- 
        pointer grabbing 
      
- 
        A client can actively grab control of the pointer, and button
        and motion events will be sent to that client rather than the client
        to which the events would normally have been sent. 
      
- 
        pointing device 
      
- 
        Typically a mouse or tablet, or some other device with effective
        dimensional motion. There is only one visible cursor defined by the
        core protocol, and it tracks whatever pointing device is attached as
        the pointer. 
      
- 
        pop-up 
      
- 
        One of the uses of a top-level shell (for example, a menu that pops up
        when a command button contact is activated). Setting the state
        of a shell to :mapped is sometimes referred to as
        mapping or popping up the shell. Setting the
        state of a shell to :withdrawn or :iconic is
        sometimes referred to as unmapping or popping down the
        shell. 
      
- 
        property 
      
- 
        Windows can have associated properties, consisting of a name, a
        type, a data format, and some data. The protocol places no
        interpretation on properties; they are intended as a general-purpose
        naming mechanism for clients. For example, clients might share
        information such as resize hints, program names, and icon formats with
        a window manager via properties. 
      
- 
        property list 
      
- 
        The list of properties that have been defined for a window. 
      
- 
        pseudo color 
      
- 
        A class of colormap in which a pixel value indexes the colormap to
        produce independent red, green, and blue values. That is, the colormap
        is viewed as an array of triples (RGB values). The RGB values can be
        changed dynamically.