Take, for instance, this great illustration on how to set a dinner table by Amy Borrell. It's informative (especially if Queen Elizabeth is popping over for dinner next week), and pretty to boot.
Another favorite, on ways to make your space appear larger, by Yumi Sakugawa.
Here's a roundup of (maybe less pretty, but equally useful) design guides from the Interwebs for you to whip out of your back pocket when needed.
1. Furniture Measuring Guide via Caroline Chair
2. Upholstery Yardage Guides via Apartment Therapy
3. Lampshade Fitting Guide via Shades of Light
4. Lampshade Buyers Guide via LampsUSA
5. Mattress Size Diagram via Penny Mustard
6. Antique Furniture Guides via Chicago Appraisers
7. Countertop Edge Guide via Houzz
8. Mirror Measurement Guide via Overmantels
9. Pattern Guide via Amy Atlas
10. Table Setting Guide via NJs Design
11. Photo Hanging Guide via A Day to Play
12. Home Office Guide via Dr4Ward
13. The Psychology of Color via Nerdgraph
14. Area Rug Size Guide via Decor Mentor
15. Cornice Style Guide via Miami Custom Drapery
16. Shade Style Guide via Miami Custom Drapery
(Images: as credited above)