Swedish curtains is the slang term for the iron window bars of a prison . The saying "to sit behind Swedish curtains" means to be in prison.
The expression was lexicographically documented as early as 1906 and goes back to high-quality iron and steel produced in Sweden .
Duden - idioms. Dictionary of German Idiomatics , 3rd, revised and updated edition, Mannheim u. a .: Dudenverlag, no year (= Duden, Volume 11), p. 694 sv "Swedish"
- Siegmund A. Wolf, Dictionary des Rotwelschen , Mannheim: Bibliographisches Institut AG, 1956, p. 110, no. 1650 "Curtains"