At some point, every builder progresses beyond the "eyeball it" method of measurement, and as you build more complex projects, the tape measure is often not precise enough. If you're assembling an engine or machining parts, for instance, you often need to be accurate to within a few thousandths of an inch or parts fail and bad things happen. Unfortunately, most of the tools that can provide this kind of precision don't survive well in a gritty, messy, all-purpose shop. Except the vernier caliper, a device that looks intimidating (especially to those who spy it in your shirt pocket) until you crack its basic code. Here's how to be as exacting as an engineer in anything you build.