The earliest finding aids were known as "calendars" and generally consisted of a listing of individual documents in chronological order, which was the preferred organizational method of historians, who were their primary users at the time. Currently used forms of finding aids vary in length and fall into several categories by type, with the inventory predominating in the late 20th and early 21st centuries.
The earliest reference resources were referred to as "calendars" and typically consisted of a chronological listing of individual documents, which was favored by historians who used them. Currently, various forms of reference tools exist, differing in length and category, with the inventory becoming more prominent in the late 20th and early 21st centuries.