One of the first things that I always go over with my students is proper use of reference surfaces for layout. When boards are perfectly machined, using reference surfaces is less critical. But when working by hand, minor inconsistencies in thickness or parallelism between faces or edges can cause major problems with joinery if the reference face and edge are ignored. That’s why one of the first videos I ever made (many years ago) was dedicated to the proper use of reference faces and edges when laying out joinery.