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Writing about centuries and decades in academic writing

9th June 2020 blog

Centuries and decades are tricky beasts. Style guides have conflicting rules, and depending on the context, additional punctuation may be necessary.

Cheat sheet

No time to read? Here’s the short version:

  • A phrase like 19th century is often misunderstood because it actually refers to the 100 years preceding the year 1900, that is, the period from 1800 to 1899. To make matters worse, the same period can also be described as the 1800s.
  • Different style guides prescribe different rules for writing centuries; there is no single ‘correct’ approach.
  • Always use numbers when referring to specific years (in 1989).
  • Don’t capitalise centuries unless they begin a sentence or form part of a title.
  • Don’t use apostrophes when referring to decades (1990s, not 1990’s).
  • Only use a hyphen if the century phrase describes a noun (twentieth-century artists or 20th-century artists).

How is a century defined?

A century is a period of 100 consecutive years, for instance, the period beginning in 1600 and ending in 1699. There are two ways to refer to a century in academic writing. The first is to refer to this period as the 1600s. The second is to call it the seventeenth century or 17th century. This second style can be confusing because it may seem odd that the 1600s and the 17th century should actually refer to the same period of time.

Words or numerals?

Different style guides prescribe different rules for writing centuries. Both the Chicago Manual of Style and the MLA Handbook recommend lower-case words for centuries (fifteenth century). The Associated Press Stylebook and APA recommend treating ordinal numbers (first, second) in the same way as cardinal numbers (one, two). Since they usually write numbers under 10 in full, the same convention is followed with centuries: if the number of the century falls between one and nine, write the ordinal number in full (ninth century); if it’s 10 or higher, use numerals (13th century).


Feudalism was the prevailing economic model in the ninth century.

Political partisanship reached new heights in the twenty-first century.


The Maya civilisation collapsed in the ninth century AD.

The 20th century witnessed two world wars.

If you’re not obliged to follow a specific style manual, pick the format you prefer and use it consistently throughout your paper.

Irrespective of the style guide, always write the century in full if it begins a sentence:

Correct:      Fifteenth-century painters increasingly embraced the use of perspective.

Incorrect:    15-century painters increasingly embraced the use of perspective.

For specific years, always use numerals:

India gained independence in 1947.

When is a century capitalised?

Centuries are only capitalised if they start a sentence or form part of a title.

Correct:      Nineteenth-century London was heavily polluted because of the extensive use of coal for heating.

Correct:      The article ‘Top Inventors of the Twentieth Century’ featured the Wright Brothers.

Incorrect:    The lute was invented in the late Seventeenth Century.

What about decades?

Decades can be just as tricky to write as centuries. You may have seen the last decade in the twentieth century written as the 1990s, the 1990’s, the 90’s, or the Nineties. Which one is right? Again, this depends on the style guide.

AP and APA require the exclusive use of numerals when writing decades, while Chicago and MLA allow both numeral and word versions, provided they are used consistently.

Generally, we recommend using numerals for decades. Use the entire four-digit number, place an ‘s’ at the end, and don’t use an apostrophe:

The 1970s were the decade of disco, flared trousers, and moustaches.

The most popular sitcom of the 1990s was Friends.

Whatever style you choose for decades, make sure it’s consistent throughout your paper.

One decade that needs extra care is the first decade of each century. Using the 1900s to refer to the first decade of the twentieth century is guaranteed to cause confusion because, as we have seen, the 1900s also refers to an entire century. In such cases, rewrite the sentence to eliminate ambiguity:

The first decade of the 20th century witnessed increasing tension among European nations with colonial ambitions.

When to use an apostrophe

You don’t need an apostrophe between the number and the ‘s’ when writing time periods.

Correct:      Grunge rock was popular in the 1990s.

Incorrect:    Grunge rock was popular in the 1990’s.

Correct:      The application of the steam engine to manufacturing led to rapid industrialisation in the 1700s and 1800s.

Incorrect:    The application of the steam engine to manufacturing led to rapid industrialisation in the 1700’s and 1800’s.

Apostrophes are sometimes used to shorten a decade from four digits to two digits (for example, the class of ’99). However, this convention is better suited to informal writing; in academic texts, use the full decade.

When to use a hyphen

If a combination of an ordinal number and the word ‘century’ is used to describe a noun (fourteenth-century warfare, 18th-century literature), it is a compound adjective and needs a hyphen:

Martin Luther King Jr. was a famous 20th-century activist and social reformer.

If the phrase acts as a noun, however, no hyphen is necessary:

The Ming Dynasty rose to power in the fourteenth century.

Not sure whether the phrase is an adjective? Try replacing the phrase with the word awesome. If the sentence makes sense with the replacement, then it is a compound adjective and you need a hyphen. If the sentence no longer makes sense, then the phrase is likely a noun phrase, and no hyphen is needed.

Modifiers are often used to describe specific parts of a century. If you use the word mid to refer to the middle of a century, you need a hyphen between mid and the first word of the phrase. Why? Because mid is an abbreviation that cannot stand alone.

California joined the Union in the mid-nineteenth century.

Many mid-twentieth-century authors and poets joined the Beat Generation movement.

Other modifiers commonly used with centuries are early and late. Unlike mid, these words can stand alone, so there’s no need to add a hyphen when they appear in the predicate of a sentence:

The Huns began to attack the Roman empire in the late fourth century AD.

You will still need a hyphen if they are used as part of a phrase that modifies a noun:

Many early-twentieth-century artistic movements, such as Dada and Expressionism, challenged the very definition of art.

Because this may leave you with an unwieldy sentence, you may want to rewrite to remove the string of adjectives:

Many artistic movements of the early twentieth century, such as Dada and Expressionism, challenged the very definition of art.

Common mistakes

Here are some of the most common writing mistakes we see during the editing process:

  • Using the wrong label for a century: nineteenth century to refer to 1900–1999.
  • Adding unnecessary apostrophes: the 1950’s or the 1800’s.
  • Adding unnecessary hyphens: in the early-16th century.
  • Forgetting necessary hyphens: 15th century art
  • Inconsistency: using both 12th century and twelfth century in the same text.

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