Need to replace your trusty old level? You can check it for accuracy. Lay one working edge on a flat surface and check the bubble. Swap it end for end, then check the bubble again. It should be in the same position. Now try the opposite working edge. Take a reading, switch end for end, and take another reading. The bubble should be in the same position. Likewise check for plumb by holding it against a flat vertical surface.
Feathering is a term which means using a brush to paint around borders and corners. When used well this technique will produce a smooth transition between the brushed parts and the larger rolled surfaces. Some painters suggest that when you feather a ceiling, for instance, you should only cut in the border a few feet ahead of where you will roll. In other words, the best way to paint a ceiling is to have one person cut (feather) in and have a second person with a paint roller come up behind. This will insure that the paint dries smoothly.