This photo demonstrates the dry fitting process of assembling a shaker style cabinet door. A trial assembly must occur prior to gluing any of the parts together. In some instances the center panel may need to be altered or perhaps you will discover that you cut the rails the wrong side for a particular cabinet door. This pre-assembly process is where you double and triple check your work.
Once you have finalized the final assembly of the shaker doors by adding the yellow glue, it will be too late when the glue hardens to make alterations to the doors size.
To watch an instructional video of the entire process go here: How To Build Shaker Kitchen Cabinet Doors