How to Write:
Centuries and Decades in Academic Documents

Writing about centuries and decades can be confusing. Depending on where you look, you’ll find variations — such as Sixties, ’60s, 1960’s, 1960s, and so on. But which version should you use?

If you’re writing an academic document, you often need to talk about the current century, since “recent” research would be in the past decade or two. So would you write the twenty-first century, 21st century, 2010s, 2010’s, or 21st Century? There are a number of options, but some of those would be incorrect. (Answer: You’d want to write the “21st century,” not spell it out — which we’ll explain more below.)

Writing the proper way is important for clarity and consistency. This guide provides comprehensive guidelines on numeral and word use, capitalization, and the correct application of apostrophes and hyphens. Refer back to this whenever you need help with writing correctly for your academic documents. Be sure to bookmark this page!

Basic Definitions

• Century: A period of 100 years. Centuries are numbered ordinally (e.g., 20th century) and refer to the years starting from 1 to 100 in each millennium.
• Decade: A period of 10 years. Decades can be referred to by their leading years (e.g., the 1990s) or by their descriptive names (e.g., the nineties).

Numerals vs. Words

• Centuries: Use words for the first through ninth centuries (e.g., the first century, the eighth century). Use numerals for the 10th century and beyond (e.g., the 20th century, the 21st century).
• Decades: Use numerals (e.g., the 1960s, the 2020s). Descriptive names can be written in words (e.g., the sixties, the twenties).

Capitalization Rules

In general, use lowercase when talking about things like “twenty years ago” or “ten years from now” or “last century.” However, you should capitalize when talking about an era that is a proper noun.

• Centuries and Decades: Do not capitalize the names of centuries and decades unless they are part of a proper noun (e.g., “the Roaring Twenties”).

Punctuating Decades

Do you use an “s” when talking about decades? Many writers err here. It is extremely common to use an apostrophe (e.g., 1950’s), but that is incorrect. When talking about decades, you are not talking about possession but a plurality. So do not use an apostrophe.

• Plurals: Add an “s” to indicate the plural form without an apostrophe (e.g., the 1980s, not the 1980’s).
• Descriptive Names: Descriptive names for decades (e.g., the eighties, the nineties) are not capitalized unless they start a sentence.

Hyphenating Years

To begin with, know the difference between a dash, en dash, and em dash. Do not use a long punctuation mark but the shortest (a simple – rather than – or —).

• Hyphenate: When centuries and decades are used as adjectives, hyphenate them (e.g., a mid-19th-century painting, a late-1970s fashion).
• No Hyphenation: Do not hyphenate when they are used as nouns (e.g., the 19th century was a time of change).

Apostrophe Usage

In English, there is a technique to shorten words, and that same technique can be used for dates. For example, we can use an apostrophe to show that some letters are missing. Thus, if shortening a date, you can do the same. Examples are below.

• Omission of Century Numbers: Use an apostrophe to indicate the omission of century numbers in decades (e.g., the ’80s for the 1980s).
• Possessives: Avoid using an apostrophe for possessive forms of centuries and decades (e.g., 1990s’ trends).

Specific Years with Examples

For specific years, always use numerals rather than spelling out. For example, never write it out like seventeen-seventy-six. Instead that would be written as 1776.

An exception would be when a year starts a sentence. In that case, you would write out rather than using a numeral. 

Correct: Twentieth-century theologians grappled with the challenges of modernity.

Incorrect: 20-century theologians grappled with the challenges of modernity.

If there’s no requirement to adhere to a particular style guide, choose a consistent format for your document.

Examples

APA/AP

• • The Enlightenment, a significant philosophical movement, began in the 18th century.
  • The 20th century witnessed groundbreaking advancements in genetic research.

Chicago/MLA

• • The concept of existentialism became prominent in twentieth-century philosophy.
  • The eighteenth century marked the emergence of modern democracy.

Turabian

• • Turabian style typically follows the rules of the Chicago Manual of Style, so centuries would be written in lowercase words (e.g., “twenty-first century”).
  • The nineteenth century saw the rise of industrialization and urbanization.

SBL (Society of Biblical Literature)

• • SBL style generally aligns with Chicago style for centuries, using lowercase words (e.g., “second century”).
  • The first century CE was a pivotal period in the development of early Christian thought.

Various style manuals have distinct approaches to handling centuries. The APA Publication Manual and the Associated Press Stylebook advise treating ordinal numbers (e.g., “first,” “second”) similarly to cardinal numbers (e.g., “one,” “two”). Following the common practice of writing numbers under 10 in words, these guides recommend spelling out ordinal numbers for centuries between one and nine (e.g., “eighth century”) and using numerals for the 10th century and beyond (e.g., “20th century”). The Chicago Manual of Style, the MLA Handbook, Turabian, and SBL generally suggest using lowercase words for centuries (e.g., “nineteenth century”).

Overview

In academic texts, centuries should be written in words for the first through ninth centuries and in numerals thereafter. Decades should be written in numerals, with descriptive names written in words. Both centuries and decades should not be capitalized unless part of a proper noun. Apostrophes should be used sparingly, mainly to indicate the omission of century numbers in decades. Hyphenate centuries and decades when used as adjectives, but not when used as nouns.

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