What does "don't pin a tree house beam" mean?

Question from George, Glen Ridge, NJ:

"In your list of things not to do, you say not to "pin" a beam. What exactly does that mean?"

Well, "pinning" would mean any way of fastening a tree house beam directly against the bark of the trunk, such that the beam will not move independently from the trunk.

This is typically done by buying the biggest lag bolts at the local hardware store (usually 1/2"), predrilling, and lagging right through the beam and into the tree.

Some tree house builders in Japan used to pin two parallel beams to a tree by clamping them together with all-thread, washers, and nuts. They squeezed the tree so hard that it would actually hold some weight before slipping down. I still consider those beams pinned because they can't move, and the tree must decide whether to push them away or grow around them - neither of which is good for the tree or the tree house.