This document provides an overview of key concepts for studying history, including:
1) History is the study of the past using evidence to understand what happened and why. Historians sequence events and interpret sources to draw conclusions.
2) The Gregorian calendar system established a standardized way to date historical events using the BC/BCE and AD/CE eras.
3) Historians use primary sources created during the time being studied, as well as secondary sources like books, to investigate the past from different perspectives. Primary sources include written documents, images, artifacts, and oral histories.