The Chicago Manual of Style, informally known as Chicago or CMS, is the reference guide for editors of fiction and nonfiction books. It’s our go-to source for editing guidelines and for any suggestions we editors make to authors about their manuscript. Although CMS contains volumes of principles and guidelines about writing, here are five general rules that might surprise you.

1. Use Numerals or Spell Out Numbers? According to CMS, you spell out numbers from zero to one hundred: zero, ten, twenty, thirty-five, one hundred. Then, starting with 101, you use the numeral: 101, 203, 5,635, etc. If you’ve seen numbers expressed differently, it’s because journalists or scientists, for example, usually follow the rule of spelling out only single-digit numbers (one, two, three, four, five, six, seven, eight, nine) and using numerals for all others (10, 20, 33, 105, etc.). The exception to the CMS rule of spelling out numbers from zero to one hundred occurs when you write a paragraph with “mixed numbers”—some above 101 and some below 100. In that case, you should express all the numbers in that paragraph as numerals.
2. Percent or %? With percentages, express the number as a numeral and spell out the word “percent”: The younger students finished 80 percent of their homework. The exception occurs when the percentage starts a sentence. Then the number should be spelled out:  Seventy-five percent of the runners crossed the finish line.
3. No Comma with a Compound Predicate. When dealing with commas, people often get tripped up with compound predicates, a sentence that contains a subject and two verbs. They see the conjunction in the sentence (and, but, or), and they want to put a comma in front of it—the way you would with two independent clauses. When in doubt, count the subjects. If there’s only one subject, resist inserting the comma: She went to the beach and read an entire novel in one afternoon (compound predicate; no comma needed). She went to the beach, and he carried the umbrella (two independent clauses joined by a conjunction; comma is needed).
4. When to Capitalize a Person’s Title. Writers often want to use initial caps with a person’s title, but CMS states that you should only use initial caps when the title precedes the person’s proper name. For example, you would be “General Green Jeans” but “Mr. Green Jeans, the general in my private army, provides good cheer to my troops.” People break this rule all the time, probably because they feel rightfully proud of their work, so they want to express that pride using an initial cap. But a good editor will stay faithful to CMS and point out this rule to authors.
5. To Hyphenate or Not. There’s a magic number in CMS: 7.85. That’s the section on compound hyphenation. CMS provides a multipage table that will answer any question you might have about whether a word should be hyphenated or not. 

Just as language changes over time, CMS also changes with the times. The new CMOS 17 edition is out now. I’m all right with no longer hyphenating “email,” but “internet” (not capitalized) still looks funny to me. However, as an editor, I need to stay current and stick by CMS rules. Umpires in baseball drive me crazy with their customized strike zone. The players should know from the start of the game how things will be evaluated. CMS allows us as editors to abide by and explain a uniform policy to authors. In other words, The Chicago Manual of Style is an editor’s conscience, guide, and companion, one that holds us accountable to our authors and publishers.