- Spell out if first through ninth: third century, seventh century.
- Use numerals for 10th and greater: 13th century, 19th century.
- Hyphenate when used as an adjective: eighth-century text, 20th-century poetry.
- We recommend using C.E. (“of the Common Era”) and B.C.E. (“before the Common Era”) rather than A.D. (anno Domini, “in the year of the Lord”) and B.C. (“before Christ”).