Thank you for the explanations. Very helpful.
Hello RailedRobin
Thanks for your hint.
Do I understand this right? The middle (of three) sector has four linedefs.
Two of them are for the connection with the bottom, the other two are for the ceiling.
What do you provide as arguments for the middle sector. In GZDB I can select "bottom", "ceiling" and "both".
Thanks in advance
zhd