MiD,
Welcome to the forum and Archicad.
Firstly, please try to post in a relevant section of the forum and not just the 'Archicad+' section.
The section descriptions will hopefully help you choose the correct one.
By way of further explanation, if adjoining elements have the same surface material and their surfaces are in the same plane, then they will join seamlessly - no joining line - as they probably would appear in real life.
You will need to two different surface materials to see a join.
If you really need the same surface and still have a line, then you can duplicate the surface (in the surface settings dialogue) and give the second one a slightly different name.
Then use those surfaces.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11