Levels and plumb bobs indicate whether surfaces are horizontally even (level) and vertically straight (plumb). Box, I-beam, and torpedo levels use spirit-filled bubble vials to measure level and plumb. Digital levels display numeric measurements of angle and slope that indicate whether surfaces are level and plumb. Optical levels, transit levels, and theodolites measure elevation differences between two points to transfer level over long distances in site surveying and construction applications. Bullseye, pocket, line, and fence post levels are compact for transport and taking readings where other levels are too large or difficult to use. Plumb bobs hang down from above to find a true vertical reference line and transfer center to the surface below.