Erroneously, it was once assumed that ''Oslo'' meant "the mouth of the Lo river", a supposed previous name for the river Alna. However, not only has no evidence been found of a river "Lo" predating the work where Peder Claussøn Friis first proposed this etymology, but the very name is ungrammatical in Norwegian: the correct form would have been ''Loaros'' (cf. Nidaros). The name ''Lo'' is now believed to be a back-formation arrived at by Friis in support of his idea about etymology for ''Oslo''.