Should I write “20th century” or “twentieth century”? Is it okay to put “the 1990s” or should I write “the nineties”? With a bewildering range of examples on the internet, in books and in periodicals, simple answers to questions about style can seem elusive.
Anyone writing or editing papers, articles, books or poetry for publication in American English, should know that the authoritative guide is the Chicago Manual of Style (CMS). This explains the conventions for punctuation, spelling, the setting out of quotations and dialogue, the presentation of abbreviations, captions for illustrations, the content of tables, and indexing. In summary, the Chicago Style covers all aspects of copy-editing.
The CMS is available in hard copy and on the web. Published by the University of Chicago Press since 1906, it is periodically reviewed and is currently in its 17th edition. All serious writers should consult it and keep abreast of developments in the latest edition.
With regard to managing citations and referencing in academic writing, the CMS will accommodate either the MLA or the APA style, its overriding concern being clarity and consistency. Your particular style guide may also specify aspects of presentation and punctuation, and you need to be aware of these. Nevertheless, you will probably find that they are compatible with the Chicago Style.
So what does this venerable guide say on the subject of dates, centuries and decades? The CMS is quite clear that particular centuries are spelled out and lowercase, “the twentieth century,” or
“the twenty-first century.” With decades, you have some flexibility. You can write “the nineties,” or “the 1990s.” If you’re being informal, you can put “the ’90s” (but be sure to get the apostrophe the right way round). Under no circumstances write “the 1990’s.” That is incorrect.
It is important to consult the Chicago Manual of Style because it covers all technical aspects of writing.