Secondary sources are sources that provide information about a topic that has already been studied or researched. They are typically written by someone who is not directly involved in the original research. Examples of secondary sources include textbooks, encyclopedias, articles in journals or magazines, and websites.
Primary sources are sources that provide firsthand information about a topic. They are typically written by someone who was directly involved in the original research or who has firsthand knowledge of the topic. Examples of primary sources include research articles, letters, diaries, interviews, and photographs.
Personal letters are considered primary sources because they provide firsthand information from the writer about their own experiences or thoughts.