from this thread suggested by the forums FAQ:

Quote: quote: What types of scene graph nodes are usually used? The general SG is a Kd-tree with a variable number of children per node. The implementation often relies heavily on polymorphism and inheritance. In fact, OOP is extremely well suited to represent an abstract SG.

if scene graph is a logical structure, how can it's nodes be kd-trees (which AFAIK are a form of spatial structure, ie. axis-aligned planes that subdivide space...)

scene graphs are not a kD-trees, it is not a spatial partitioning structure but can contain spatial information, don't let "only logical relationships" fool you scene graphs can still contain spatial info but its not a spatial partitioning structure like a kd-tree. For example there can be transform nodes in a SG that is spatial information is it not?

A scene graph represents the semantical and/or spatial relationships (not spatial partitioning) among objects in a scene. Its structure is generally arranged in a hierarchical manner.

Scene graphs are typically restricted forms of directed acyclic graphs (DAGs) or n-ary trees. Most scene graphs are typically an instance of the composite design pattern and are augmentated bounding volume hierarchies (BVH) (which are object partitioning structures by the way).