The "standard" plural of Bogen is Bogen, but Bögen is used very widely, too (apparently now everywhere in Germany, not only in the South), and there's practically no difference between them. In some compounds you can use only -bogen, e.g. Ellenbogen (Ellenbögen would be wrong). http://canoo.net/blog/2008/09/08/die-bogen-und-die-bogen/ http://gfds.de/bogen-plural-von/