The use of hyphen in "twentieth-first-century reader" and "twentieth-first century reader"

Question

Which variant is correct: "twentieth-first-century reader" or "twentieth-first century reader"? Is the use of the second hyphen in the first example a matter of taste or not?

Answer 1

Ordinal numbers — those that order things first, second, third, erc. — are formed after even tens by making only the second number an ordinal:

forty-fourth president, twenty-first birthday, seventy-fifth anniversary

As for the hyphenation, the Chicago Manual of Style recommends hyphenation with numbered centuries when used attributively, i.e., before the noun modified, but not predicatively, i.e. after a linking verb:

century:

• the twenty-first century

• the twenty-first century

• fourteenth-century monastery
• twenty-first-century history
• a mid-eighteenth-century poet
• late nineteenth-century politicians
• her style was nineteenth century

Many writers follow this recommendation:

No single modern English word will convey all of these meanings to a twenty-first-century reader, and substituting a phrase may destroy the intended ambiguity.

… but it will provide a list of novels and non-fictional works that may have been missed, even by the most conscientious twenty-first-century reader.

And yet, while the distinction appears clear enough in principle, and is no doubt familiar to the twenty-first-century reader, Goethe's essay reveals a striking complexity in terms of its practical application.

Is the twenty-first-century reader facing a crisis of cultural confidence like that of the author in the twentieth?

Other writers, however, do not use a hyphen before century. The reasoning is that a reader is not likely to understand, for instance, this book as one of several publications inexplicably all numbered twenty-first of a series named Century Readers:

It would be quite difficult to misconstrue the following sentences. The second hyphen is optional, i.e., a convention, not a rule.

… to entertain rather than challenge the faith of the reader, as well as to appeal to the cynical twenty-first century reader versus the transcendental nineteenth century reader.

Not only a twenty-first century reader, but also a nineteenth century reader could have pointed out just how undesirable this situation is.

As a twenty-first century reader of the 1839 Constitution we can easily see how it is time-bound—a product of its culture.

The Seren series, entitled New Stories from the Mabinogion, takes these ancient tales and retells them for the twenty-first century reader.

So unless your writing must conform to a particular stylesheet — or the preference of an instructor — you are free to choose whichever form you wish.

Answer 2

These examples come from The Chicago Manual of Style

the twenty-first century

twenty-first-century history

This means that the Chicago Manual says that if you are talking about the century itself (for example "This is the twenty-first century") you only use a hyphen between 'twenty' and 'first' but if you are talking about something which belongs to or is defined by the century (for example "Twenty-first-century cars") you use hyphens between 'twenty' and 'first' and between 'first' and 'century' so "twenty-first-century readers" is correct.

Notice though that what you wrote (twentieth-first-century readers) is not correct. One and twenty are the names of two numbers so we talk about the first customer and the twentieth century; but twenty-one is the single name of a different number so we talk about the twenty-first century or the twenty-first customer.

Words like twentieth and first are called ordinal numbers and we only need to use the ordinal form of the last part of the number. If we say that 1984 was the one thousand nine hundred and eighty fourth year of the Common Era only 'fourth' is written as an ordinal number.