In the gangway to the bedrooms in his home, Marc needed extra place for the books to read in the bedroom. The most suitable space was a corner just outside the rooms. However, the corner BILLY unit sold in IKEA would have taken up too much space.
So, he took two regular BILLY bookcases and converted one of them into a corner element.
If you join two normal BILLY bookcases in a corner you lose a lot of space in the corner which never more can be cleaned up.
His solution was using the corner in full without losing any corner space.