Follow tag in new window
From Vim Tips Wiki
created 2003 · complexity basic · author PopEsc · version 6.0
If you use tags, you might want to follow a tag, but leave the current window open. The following mapping allows you to do this easily with Ctrl-\.
:map <C-\> :sp<CR><C-]><C-w>_
This creates a new window of maximal size and the original window shrinks to one line. To have the windows be of equal size, just eliminate the "<C-w>_" at the end.
Just use 'CTRL-W ]'.