This option enables you to split the current window or frame into two
(sub)frames and to see several (parts of) documents side by
side at the same time. To make a (sub)frame larger (and the
other smaller) move the border between the frames.
By double-clicking on the page in a (sub)frame the page
in that frame will be shown in the whole window or, if you started from a
frame, in the whole original frame. However, this may not (yet) work for
special or old pages (such as this page at the top or on the left). Of
course, you can also return to an undivided window or frame by going back
as many times as you have gone forward.