From DIYinfo.org
Larger projects such as cabinets, shelves, foundations, decking or any large project can often be challenging to "square up".
These two handy tips will help you square up even the largest projects.
Opposite Corners
A quick trick to ensuring boxes, shelves, and other projects are perfectly square, is to measure from opposite corners. When the two measurements are the same, the project is perfectly square.
3-4-5 Triangle Method
This trick is used often in the construction industry for setting up foundations and other large projects. To use this method you would measure and mark a point 3' (or 3m) from the corner. Next measure 4' (or 4m) in the other direction. Stretch a tape measure between these two points. When the distance between them is 5' (or 5m) the corner is square.
