How is a 'letter' classified in sources?

A letter is classified as a primary source because it is an original document created by an individual who is expressing thoughts, feelings, or information based on personal experience or knowledge at the time the letter was written. Primary sources provide direct insight into a subject, offering firsthand accounts or evidence directly from the creator. In the case of a letter, it serves as an authentic artifact of a particular moment in time, reflecting the context, sentiments, and immediate responses of the writer.

This classification distinguishes it from secondary sources, which analyze or interpret primary sources, and tertiary sources that compile and summarize information from primary and secondary sources. Research sources can vary widely and do not specifically categorize a letter as inherently primary.