Hanging pictures or shelves requires a degree of precision. If you're having a hard time measuring in a space, a water tube level can help.
The water tube level is simple: connect two empty water bottles with a length of hose, fill them with water, and measure the height of the water inside the tube. Since the two water bottles are connected, the levels will always match. This is particularly helpful if the two points are on different walls or otherwise difficult to reach with a measuring tape.
WATER TUBE LEVEL (from scraps) [Instructables]