If you want your walls to be at perfect 90 degree angles -- and believe us, you do -- a chalk line will come in very handy. This simple contraption is a plastic or metal case that houses a retractable string covered in powdered chalk. The end of the string has a hook ring, so you can hook it to your starting point and pull away like you would with a tape measure. Once you've reached your end point, you simply pull the string tight and snap it to create a line of chalk, also called snapping a line. They're also handy for hanging wallpaper and laying ceramic tiles.