If you've got a bunch of 4x4 posts to install on a deck or fence or whatever, this tool is *sooooo* useful. It is a simple thing that wraps around two sides of a 4x4 so that you can level two planes at once. Big ol' rubber bands attach it to the side and wrap around it four inches or so, and it has three levels built into it so you can level in two directions at the same time. Guys I know who do this for a living carry at least a pair of these, if not several pairs. You just leave one strapped onto the far pole, or each post/corner of a deck, to make sure it remains unchanged while you jiggle the near one. I use one if I'm building shelves in a vertical position, or for anything that requires leveling on two planes at once. It's in that "why didn't I think of that?" category.