centuries

1.  The first century consisted of the years 1 through 100. Therefore, the second century consisted of 101 through 200, the third century 201 through 300, and so on. That’s why the 19th century, for example, consists of the 1800s instead of the 1900s.

2.  When used as phrasal adjectives, centuries are hyphenated—for example:

We found ourselves mixing with dashing Musketeers, 18th-century aristocrats with powdered faces and wigs, Madame Defarge look-alikes and Napoleonic-era duelists.

Used as a noun, a century has no hyphen—for example:

The same intuition can be found in almost all the literature of the 19th century, from Wordsworth to Dostoevsky.

3.  Centuries aren’t capitalized. According to conventional usage rules, “19th Century” is incorrect.