Auto Coder for On-Line Printing does automatic registered coding of packaging information on cartons moving on automatic conveyors or taping machines. The Auto Coder is designed for on-line printing on corrugated cartons, paper bags, cement or asbestos, which move on continuous conveyor. The Auto Coder is equipped with a tension spring that returns the marker to its zero position after every stroke. Auto Coder for On-Line Printing is provided with individual alphanumeric type to compose almost any type of information that the user wishes to mark.