An Emacs Gem is one of the many little tools/functions/modes that Emacs ships with and that turned out to be ‚extremely usefull‘ in a specific situation.
If you ever want to compare two buffers line by line and are too lazy to fire up ediff or don’t use ediff at all you might end up with a google search for „Emacs scroll windows simultaniously“. The top results will then inform you that Emacs brings a solution for this task: scroll-all-mode.
Scroll-all-mode does not need lots of explanation and if I am not mistaken the official Emacs Manual dedicates a single line of documentation (which in fact suffices):
Scroll All mode (M-x scroll-all-mode) is a global minor mode that causes scrolling commands and point motion commands to apply to every single window.
Every single window in the selected frame one might add – but that’s it. For a quick comparison it’s definetly a great tool and it comes with Emacs. No need for installation (it’s not a package). No need for a command-window (it’s not ediff).