A letter written during the time an historical event occurred would be a primary source document. Primary sources come from the period of history you are studying. For example, if this letter was written from a German husband to his wife during World War II, someone trying to study the wartime effects on German soldiers would like to read it because it would give them direct insight into what they are studying. Secondary sources, on the other hand, are books, articles, or films about the past where other historians have already analyzed primary materials. If you write about the primary source letter, your writing would become a secondary source.
