They are referred to as primary sources as they were written by someone on the scene so to speak. A secondary source would, for instance, be a book written examining a group of letters making inferences from them. A tertiary source would be an essay written from the book without reference to the letters themselves.
The difference can be described sort of as below:
Primary source:
I saw a black cat with a big white spot yesterday. Signed, your friend Sue.
Secondary source:
Some black cats were thought to have spots, letters refer to cats with spots. By Prof. A. N. Other.
Tertiary source:
Prof. A N. Other reports that all black cats have spots.
Letters, pictures, diaries.
Yes, because letters and diaries of soldiers would be considered what is called a primary source. A primary source is a document or artefact created at the time of the event. This means that letters and diaries of soldiers would be considered accurate historical evidence.
Letters and diary entries from pioneers moving west.
Patient files, closed down factories and asylums, descendants of patients, letters, diarys
Corroborated sources of historical evidence. (APEX) !/
what is the historical evidence for the foundation of rome
Evidence, secondary sources, and forgery. :)
Letters and diary entries from pioneers moving west.
Why were the letters written? -
There is no historical evidence of that nature.
Historical letters have been preserved in different forms including using videotape recorders. They can also be preserved in glass or plastic.
Historical synthesis is the process an historian engages in to transform evidence into a final historical account (O'Brien, 1935)