If you're often confused about what size hole to drill for a certain pipe size or just want to be able to communicate with vendors and fellow brewers a little better when discussing DIY projects, you really need to be able to accurately measure things. For example, 1/2" NPT threaded pipe nipples actually measure .85" OD. It's almost impossible to measure that kind of thing with a tape measure due to the curvature.

This two-pack of plastic calipers are very cost effective and will help you measure outside diameters or outside dimensions of odd shaped parts , inside diameters and cutouts, and also the depth of narrow holes. You still need to know how to read a ruler, but for 2 bucks, you can't lose.