Melvin Merzon sets me this multi-part question:
How would you write “21st Century”? In a legal document? In a business letter? In fiction? In a nonfiction context?
My short answer for all specified contexts is twenty-first century.
Unless the name of the century begins a sentence or is part of a proper name, it is written in all lowercase letters: We are living in the twenty-first century.
When a century is part of a proper name, no hard and fast rule can apply. Someone naming a program, company or a book may express the century any way they wish:
Twenty-first Century Scholars
Twenty-First Century Foundation
Twenty-First-Century Gateways (In this book title the century name has become an adjective.)
20th Century Fox
Century 21 Realty
Newspaper headline writers may also exercise freedom when writing the century: New Year Rings in 21st Century
Bottom line: go with twenty-first century unless there is some reason not to–for example, contrary guidelines in a style manual you are required to follow.
Writing the Decades
Decades may be spelled out or expressed in numerals:
NOTE: There’s no apostrophe between the numerals and the letter s.
The same rule about capitalization applies to decades as to centuries: if the decade is part of a proper name or title, it will be capitalized; otherwise leave it in lowercase. For example, write “the nineties,” but “the Gay Nineties”
Referring to the first two decades of a century can be tricky.
For example, if you want to talk about the first decade of the century, you can’t write the 1900s, or the 2000s because too many readers would assume you’re referring to the entire century.
Another problem is that not all authors agree as to what years are included in a decade. Is the “first decade” of the 1900s 1900 to 1909, or 1900-1910?
And what about the second decade? Some writers talk about the “teens” of a century, but what about the years ending in -10, -11, and -12?
When writing about the first two decades of a century, it’s probably best to be a little wordy for the sake of clarity. For example: History seemed to repeat itself in the decade 2000-2009.