A Combination Square. A Must Have Tool.
There are some tools that you should not live with out and the #combination square is one of them. Get a good quality one as it will last a life time and the cheaper ones are not as accurate.
I found this chart on it and thought it was great for understanding what it is and what you can do with it.
It is a great tool for making sure things are at a 90 or 45 degree angle. This is invaluable when you are making a box or mitre corners for a picture frame.
Most of them come with a level but will only do flat leveling and not 45 degrees. One quick tip when using this for a project on your workbench, is to make sure you workbench is level first before you check the project your are working on. If the workbench is off, then so will your project.
I also use mine as a depth finding tool. Set the blade into the hole and tighten it up. You can then check the same depth on the next hole.
Along with using this as a depth finder you can also use it to mark a cutting line down a piece of wood. Move the blade to the desired length and then just run it along the wood with a pencil or scribber.
This is a must have tool and no tool box should be without a Combination Square.
Source: Fix.com Blog