Updated August 18, 2026
CMOS 9.35 in the Spotlight
This post was originally published in 2019, or just before the beginning of the 2020s, also known as the ’20s or the twenties. Every decade after this one will be referred to similarly: 2030s, ’30s, or thirties; 2040s, ’40s, or forties; and so on. Any of those forms would be considered correct as long as you maintain some consistency.
But how should we refer to the two decades that we’ve now left behind?
The Rule and Its Consequences
According to paragraph 9.35 in CMOS 18, “Decades are either expressed in numerals or spelled out (as long as the century is clear) and lowercased.”
For 80 percent of any century, that’s easy. Simply refer, for example, to the 1920s* or the twenties, the 1930s or the thirties, and so on, up to and including the 1990s or the nineties. The same holds true for this century. Or you can abbreviate the numbers and refer to the ’20s and ’30s and so on. Just be consistent, and be sure to get the apostrophe right (’20s, not ‘20s).
The Trouble Begins at the Beginning
Unfortunately, the forms that work so well for referring to the third through the tenth decades of a century aren’t fully applicable to the first two.
The first decade of any century is the most difficult one to write about. Turning to CMOS 9.35 again: “The first decade of any century cannot be treated in the same way as other decades. Though it commonly appears in journalism and may be clear from the immediate context, ‘the 2000s’ could easily be taken to refer not to a decade but to the whole of the twenty-first century (likewise for the 1900s, the 1800s, etc.).”†
“The 2000s,” however, is a tempting shorthand. That term has become especially common in journalism, where the focus is often on the recent past.
It’s also used at Wikipedia, which includes an entry for “2000s” that defines the term as follows: “The 2000s (pronounced ‘two-thousands’; shortened to the ’00s) was the decade that began on January 1, 2000, and ended on December 31, 2009.”
Wikipedia’s entries for centuries include the word “century”: “21st century,” “20th century,” “19th century,” and so on. This helps to differentiate these entries from the entries titled “2000s” and “1900s,” which, again, refer to decades; such decades from earlier centuries each carry a label: “1800s (decade),” “1700s (decade),” and so on.
So, at Wikipedia, “1700s” primarily means 1700 to 1709, not 1700 to 1799, as a literal reading of the expression would require. A search at Wikipedia for “1700s” directs the reader to the articles for the 18th century and the 1700s (decade).‡
That approach may work for an online encyclopedia, where entry terms are defined and explanations are always a click away. But it’s not Chicago style. In Chicago style, “the 1700s” is synonymous with the eighteenth century, and “the 2000s” refers to the twenty-first century (see also CMOS 9.34).
Applying Chicago Style
To refer to the first decade of a century in Chicago style, you need to be either more explicit or more precise. This means either spelling it out or writing it in the form of a numeric range; then, once the century has been established, the first decade can be referred to as such, or you can use a two-digit short form:
the first decade of the twenty-first century or 2000–2009; then the first decade or the ’00s (or, informally, the aughts)
but not
the 2000s (except in references to the whole century)
Not that writing “the 2000s” to refer to ’00–’09 (to use another shorthand) is wrong; as suggested above, that form can be useful in journalistic contexts for referring to what is still recent history. But Chicago takes the long view. Many decades from now, when the years 2000 to 2009 have become a distant memory, “the 2000s” will be more likely to evoke the century than the decade.
The Second Decade
The second decade of a century, like the first, resists being named. You could try referring to “the teens” or “the tens,” but neither term is sufficiently precise. The first expression glosses over the fact that the numbers ten through twelve (as in the years 2010 to 2012, 1910 to 1912, etc.) aren’t in the teens; the second one ignores the fact that the word “ten” applies only to 2010, 1910, and so on.
But there’s no ambiguity about the full numeric form of a second decade. So, to follow Chicago style, write “the 2010s” (or “the ’10s” for short).
Conclusion
Now that we’re well into the 2020s, remember that terms like 2000s and 1900s and so on will always need additional context to be understood as decades; to historians and others who follow Chicago style, those expressions are best reserved for centuries.
* In Chicago style, the plural of a number is formed with an s alone (no apostrophe).
† The 2000s could also, of course, refer to the third millennium, but millennia are more typically spelled out.
‡ Note that Wikipedia defines “18th century” as the years 1701 to 1800 but “1700s” considered as a century as the years 1700 to 1799. Ditto for other centuries.
Photo of US$20 bills by Nic McPhee, licensed under CC BY-SA 2.0.
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