Basicly I want to make a gab in a plane.
I looked it up here but all I have found didnt helped me out 
Also I founded ‘building a castle’. Now I can make a door but not a window. It just don’t work for me.
I’m building my first blender image: A musuem (schoolproject) and now I want to ad windows to it.
So I have to make the gabs in the walls (walls are planes which I extruded)
how can I do this?
I know that link: already read it.
But i can’t find that ‘intersect’ button in editmode (F9)
And how do you join 2 meshes?
‘Intersect’ button was removed in 2.26.
Now we have booleans (make a search in this forum)
Stefano
For booleans get the pdf downloadable at www.blender.org in the documentation section (not the documentation forum).
Join = Ctrl-J
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I’m confused
If I select 2 meshes and Join them…end press W --) Intersect. It says that I have to select 2 meshes.
If I select 2 different meshes. The only thing that happens is, is that there is draw a diagonal line:
And if I turn to shaded mode: There is no gab at all 
Whell I added a screenshots to make it more clear: I just dont understand it.
You would be better off planning (or drawing) your model ahead of time so you know where you want your windows to be located. Then model the openings for the windows as you create the walls. Sometimes I will even model a plane using curve circles (set to poly) which is then very easy to add “holes”. Moving the holes around is very simple and the when done, convert to a mesh.
Working with Booleans can be an exercise in frustration.
My 2 cents…
OK, you want to merge two meshes into one. Select the loops (rows of verts) that will be merged. Grab and move the meshes so that they line up at the interface with the verts as close to one another as possible. Ctrl-J. Remove doubles.
The problems: If the verts in the two respective loops don’t correspond (ie, they are different in number and relative spacing) the doubles won’t all be removed.
To prepare for removing the doubles it is best to scale them down to the closest possible tolerances possible. That’s easy with individual verts but tricky at best with loops.
An alternative is to position the two meshes close to one another, Ctrl-J join them and then build polys between them (select two verts from opposite meshes and hit F to make a new edge, repeat with next two, then select all four and hit F to make a face).
The best advice is from Curtis… Plan!
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It worked…but it took me a bit longer than the way you explained. (That I tried but…)
I just made meshes which where the windows. Then put the meshes around it…finally I joint the meshes (without the meshes wich where the windows)
Now I have gabs where my windows can fit in 
Tnx…need to plan a lot more…indeed.
