Hi Burr, normally the case where the edge is solitary and only on one face is not considered non-manifold (like in your second image).

But yes the first example you have there is a good one, where having those 2 boxes combined together into one structure would require that one edge to be shared between 4 faces instead of only 2 faces.

Internally the geometry library that MoI uses allows for such things and it tends to help to make booleans work better since things can be combined all together into one unified structure before it focuses on eliminating different parts of it.

There is a kind of normalization process that MoI does to any non-manifold results after the calculations are done, to make sure they are split up into different manifold chunks.

The equivalent for faces is having it possible to have one structure that has a face in common between 2 different solid regions, like this:



- Michael