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(elisp)Basic Windows

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Basic Concepts of Emacs Windows
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   A "window" in Emacs is the physical area of the screen in which a
buffer is displayed.  The term is also used to refer to a Lisp object
that represents that screen area in Emacs Lisp.  It should be clear
from the context which is meant.

   Emacs groups windows into frames.  A frame represents an area of
screen available for Emacs to use.  Each frame always contains at least
one window, but you can subdivide it vertically or horizontally into
multiple nonoverlapping Emacs windows.

   In each frame, at any time, one and only one window is designated as
"selected within the frame".  The frame's cursor appears in that
window.  At any time, one frame is the selected frame; and the window
selected within that frame is "the selected window".  The selected
window's buffer is usually the current buffer (except when `set-buffer'
has been used).  Note: Current Buffer.

   For practical purposes, a window exists only while it is displayed in
a frame.  Once removed from the frame, the window is effectively deleted
and should not be used, _even though there may still be references to
it_ from other Lisp objects.  Restoring a saved window configuration is
the only way for a window no longer on the screen to come back to life.
(Note: Deleting Windows.)

   Each window has the following attributes:

   * containing frame

   * window height

   * window width

   * window edges with respect to the screen or frame

   * the buffer it displays

   * position within the buffer at the upper left of the window

   * amount of horizontal scrolling, in columns

   * point

   * the mark

   * how recently the window was selected

   Users create multiple windows so they can look at several buffers at
once.  Lisp libraries use multiple windows for a variety of reasons, but
most often to display related information.  In Rmail, for example, you
can move through a summary buffer in one window while the other window
shows messages one at a time as they are reached.

   The meaning of "window" in Emacs is similar to what it means in the
context of general-purpose window systems such as X, but not identical.
The X Window System places X windows on the screen; Emacs uses one or
more X windows as frames, and subdivides them into Emacs windows.  When
you use Emacs on a character-only terminal, Emacs treats the whole
terminal screen as one frame.

   Most window systems support arbitrarily located overlapping windows.
In contrast, Emacs windows are "tiled"; they never overlap, and
together they fill the whole screen or frame.  Because of the way in
which Emacs creates new windows and resizes them, not all conceivable
tilings of windows on an Emacs frame are actually possible.  Note:
Splitting Windows, and Note: Size of Window.

   Note: Display, for information on how the contents of the window's
buffer are displayed in the window.

 - Function: windowp object
     This function returns `t' if OBJECT is a window.