Ship lap joint and nail pattern
 
Ship lap joint and nail pattern

This is the board I start nailing first, shown by the nail holes on the right which aligns to the edge of the board that meets the cupboard face. The second board will slip under the lap tightly to begin nailing. As I stated earlier, each board is planed perfectly square and each shelf edge has a penciled center line that corresponds to the score lines marked for the nailing pattern. Nothing willy-nilly here ! Cool huh !