The main sources of information about the Norse voyages to Vinland are two Icelandic sagas, the "Saga of Erik the Red" and the "Saga of the Greenlanders", which are known collectively as the Vinland Sagas. These stories were preserved by oral tradition until they were written down some 250 years after the events they describe. The existence of two versions of the story shows some of the challenges of using traditional sources for history, because they share a large number of story elements but use them in different ways. A possible example is the reference to two different men named Bjarni who are blown off course. A brief summary of the plots of the two sagas, given at the end of this article, shows other examples.