Today, there are many methods of interlocking joints in woodworking, but for the most part, they all require some form of an additional attachment. Wood glue is traditionally used to create laminar beams and other connections, but with a perfect system of joints, nothing else is needed. That's where the scarf joint method, or kanawa tsugi (金輪継), enters.
Before there were complex wood glues and screws, a specially trained class of Japanese carpenters called miyadaiku (宮大工) used a technique of interlocking wood joints to connect wood to build sturdy structures. Tasked with building and maintaining shrines and temples, they used wood joinery to directly connect pieces of wood using a technique known as kanawa tsugi.
Later on, these craftsmen went on to build some of the most long-lasting structures of Japan using kanawa tsugi. Back in the day, almost a thousand years ago, people in Japan had a hard time accessing iron. Hence, they had to rely on their skills to make up for what was missing in materials, eventually turning their work into beautiful art pieces that stand the test of time.
The engineering behind it? The system of interlocking joints is engaged using a wedge, which is hammered into open slots left in the wood by design. This method does require quite a bit of detailed woodworking, but when glues or screws are not an option, these joints are perfect. Not to mention, joining wood in this manner places congruent strain on the beam, and it does not create potential failure shear planes that can be present with other glue joining techniques.
Joining beams like this would have been the traditional method used in all Japanese architecture at the time. This would have allowed them to create larger spans across roofs and create taller structures without the need for additional supports.
Building techniques have evolved quite a bit thanks to the modern industrial revolution, but there are still groups out there, perpetuating the beautiful old methods of construction that are in sync with nature.