Typography rules for writers

As a pro­fes­sion­al writer, you should hold your doc­u­ments to the same stan­dards as pro­fes­sion­al­ly pub­lished ma­te­r­i­al. Why? Be­cause your doc­u­ments are pro­fes­sion­al­ly pub­lished material. More­over, much of what writ­ers con­sid­er prop­er ty­pog­ra­phy is an ac­cu­mu­la­tion of bad habits and ur­ban leg­ends. These will be set aside in fa­vor of pro­fes­sion­al ty­po­graph­ic habits. Any writer can mas­ter the es­sen­tials of good typography.

... from John Tranter, Australian poet and publisher
Very useful: http://practicaltypography.com/summary-of-key-rules.htmlSummary of key rules

1. The four most im­por­tant ty­po­graphic choices you make in any doc­u­ment are point size, line spac­ing, line length, and font (pas­sim), be­cause those choices de­ter­mine how the body text looks.
2. point size should be 10–12 points in printed doc­u­ments, 15-25 pix­els on the web.
3. line spac­ing should be 120–145% of the point size.
4. The av­er­age line length should be 45–90 char­ac­ters (in­clud­ing spaces).
5. The eas­i­est and most vis­i­ble im­prove­ment you can make to your ty­pog­ra­phy is to use a pro­fes­sional font, like those found in font rec­om­men­da­tions.
6. Avoid goofy fonts, mono­spaced fonts, and sys­tem fonts, es­pe­cially times new ro­man and Arial.
7. Use curly quo­ta­tion marks, not straight ones (see straight and curly quotes).
8. Put only one space be­tween sen­tences.
9. Don’t use mul­ti­ple word spaces or other white-space char­ac­ters in a row.
10. Never use un­der­lin­ing, un­less it’s a hyperlink.
11. Use cen­tered text sparingly.
12. Use bold or italic as lit­tle as possible.
13. all caps are fine for less than one line of text.
14. If you don’t have real small caps, don’t use them at all.
15. Use 5–12% ex­tra let­terspac­ing with all caps and small caps.
16. kern­ing should al­ways be turned on.
17. Use first-line in­dents that are one to four times the point size of the text, or use 4–10 points of space be­tween para­graphs. But don’t use both.
18. If you use jus­ti­fied text, also turn on hy­phen­ation.
19. Don’t con­fuse hy­phens and dashes, and don’t use mul­ti­ple hy­phens as a dash.
20. Use am­per­sands spar­ingly, un­less in­cluded in a proper name.
21. In a doc­u­ment longer than three pages, one ex­cla­ma­tion point is plenty (see ques­tion marks and ex­cla­ma­tion points).
22. Use proper trade­mark and copy­right sym­bols—not al­pha­betic approximations.
23. Put a non­break­ing space af­ter para­graph and sec­tion marks.
24. Make el­lipses us­ing the proper char­ac­ter, not pe­ri­ods andspaces.
25. Make sure apos­tro­phes point downward.
26. Make sure foot and inch marks are straight, not curly.