Level spelled backward is level,as both ends of a foundation should be! Bubble levels come in many shapes and sizes, the smallest one used to level the longest length. It is a line level and the idea is to stretch a string as taut as one dares to and hang this little 3 inch level on it. These are hard to work with and wind can cause false readings. The best level for long foundations is the water level. Other builders levels are the torpedo, a 9" level, that are great for scaffold building if they are magnatized. There are 18", 2ft,4ft, and 6ft levels. It is always a good idea to check one level against another by simply stacking one on top of the other to make sure that they read the same.