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Century
Historical Eras
Centuries categorize history into 100-year blocks to simplify reference to specific eras. 
Ordinal Numbers
When talking about centuries, use ordinal numbers like 20th, 21st to describe sequential order. 
Common Misunderstanding
The 20th century refers to the years 1900-1999, not 2000-2099, as might be commonly misunderstood. 