What do primary sources include?

What do primary sources include?
Primary sources are the documents themselves.
If you want to study the constitution you could read the constitution (primary source) or you could read a scholarly essay about the constitution (secondary source).

It is important to research using primary sources because the middle man in the secondary source could be biased or inaccurate. Preservation of primary sources allows for the constant reevaluation of current interpretations present in secondary sources.