Writing about historical periods of time is an important part of genealogical compilation. Knowing how to use these terms properly is critical. When referring to a period in time that is defined numerically, the number is lowercase unless it is a proper name. For example, one would write twentieth-century, but would properly write the Fourth Republic.
Capitalization rule are also important for descriptive periods. Once again, descriptive terms are lowercase unless referring to a proper name. Examples include the antebellum period, colonial era, and Victorian era. Note that even when capitalizing the proper name of Victorian (after Queen Victoria), the word era remains lowercase.
Some terms have come to traditionally refer to a certain period of time, and are capitalized by tradition. Examples include the Counter-Reformation, the Middle Ages, the Roaring Twenties, and the Gay Nineties.
Historical events can provide a challenge. According to the Chicago Manual of Style, 16th edition (CMS): “Names of many major historical events and programs are conventionally capitalized. Others, more recent or known by their generic descriptions, are usually lowercased. If in doubt, do not capitalize.” (CMS 8.74). For more information and detailed examples about dealing with historical events and periods, consult CMS 8.70–8.83.