Building boxes is probably one of the fundamental tasks in most woodworking.  Want to build a chest?  Its a box filled with other boxes.  A dining table?  A box on legs.  A Morris Chair?  A trapezoidal box.  Get the point?  Making cases and the accuracy required to do it is extremely important.  

But is this accuracy all about measuring stuff and making batch cuts at the table saw?  What if you want to build by hand?  Heck, what if you just don't want to get caught up in the fractions and precision created by casework.  A hand tool approach can create highly accurate casework without a ruler ever entering into the picture.  The key is understanding what your variables are and how to convert variables into constants.  

This is starting to sound a lot like an algebra class but I guarantee that if you follow this approach you will take the stress and uncertainty out of accurate boxes and find yourself a better and more efficient woodworker able to make boxes that fit inside boxes that fit inside boxes...even if those boxes aren't supposed to be perfectly square!