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 to display
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 Buffer Display Action Functions) 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).