A. That example in chapter 8 is intended only to illustrate that when a decade or century is written in words, such an expression isn’t capitalized. Our usual preference would be for numerals (“1800s”), but either form is acceptable (choose one and be consistent). Note that Chicago considers “1800s” to be equivalent to “nineteenth century”—which also happens to be the more common way of expressing a century in words. (Under Chicago’s alternative rule for numbers, according to which numerals are used for numbers greater than nine, it would be “19th century”; see CMOS 9.3.) We should also note that in Chicago style, “1800s” and “1900s” refer to the whole century, not just the first decade. For more, see our post on decades at CMOS Shop Talk.