Without a tool that shows you level and plumb you can't build a house that works well or looks good—not to mention hang kitchen cabinets or trim out a door. The most basic such tool is a spirit level, which indicates level and plumb with a bubble inside a liquid-filled vial. At the high-tech end are laser levels, which project perfectly level or plumb lines onto a surface. Laser tools are great for layout work, but their fleeting lines aren't much help when you have to physically check the positioning of an object. For that, you need a tool with a bubble vial, which can rest on your workpiece or butt up against it. Only then can you be sure your work is on the level.