In 1967, Yamaha introduced their first drum set - the D20 and D30. From their first conception, these drums were made from birch. The ends of the master ply are cut diagonally, ran through the glue machine and then put into the mold. Once they overlap the diagonal unto itself they hit the joints so they are locked in. Each seam is staggered 120 degrees so that the tension of the shell is evenly spaced. Once this is done, they stick an airbag in the middle of the shell so the glue is evenly spread throughout the drum. This method doesn't require the wood and glue to be put under high temperatures to create a bond.