Side windows are special windows positioned at any of the four sides of a frame’s root window (see Windows and Frames). In practice, this means that the area of the frame’s root window is subdivided into a main window and a number of side windows surrounding that main window. The main window is either a “normal” live window or specifies the area containing all the normal windows.
In their most simple form of use, side windows allow displaying
specific buffers always in the same area of a frame. Hence they can
be regarded as a generalization of the concept provided by
display-buffer-at-bottom (see Action Functions for Buffer Display) to the remaining sides of a frame. With suitable
customizations, however, side windows can be also used to provide
frame layouts similar to those found in so-called integrated
development environments (IDEs).