Dave is right.
Volumes meeting at a common edge are not regarded as 'solid'.[image: Vq8i_Capture1.PNG]
Volumes that are separate are 'solid'.[image: BgbM_Capture2.PNG]
Volumes that overlap and share a common 'internal' face are not regarded as 'solid'.[image: H4oy_Capture3.PNG]
Volumes that overlap but have no internal faces [here they were removed] are 'solid'.[image: oZp3_Capture4.PNG]