Hi Pierre, it looks like your problem is your extrusions are not actually solids, they are open at the top and bottom. The main way this would happen is if you unchecked the "Cap ends" option in the Extrude command:



By default "Cap ends" should be checked - maybe you were experimenting with the options and turned it off?


> (Question: Do I have to first make a planar (Construct->Planar) out of my rectangle to get a "real" solid?)

Nope, it's not necessary to use Construct > Planar first before doing the extrusion, you can generate a solid directly from the rectangle as long as "Cap ends" is turned on.

So I'd recommend deleting everything there except your 2 rectangles, doing the extrusions again making sure "Cap ends" is checked, and then do the boolean. One thing to note is that it can indeed be a little tricky to select the inner solid when doing the boolean because it's blocked from clicks by the larger object. One way to do it is to use the Edit > Hide button while you are in the boolean command and it's asking you to select objects to subtract. That will then hide the outer object so it's out of the way and you can see the inner one to click on it more easily. Another way you can select objects contained within others is to use window select - that's where you click the mouse down in empty space and drag out a selection window. To do that you'd probably want to hide or delete your generator curves so they won't be captured in the window selection either, if you end up selecting one of those curves as the cutting object it will cut a hole all the way through.

Hope that makes sense, please let me know if you're still stuck.

- Michael