These levels have ground and graduated main vials. All sizes have a cross test vial except the 4" (100mm) model. The 12" (300mm) model also has a plumb vial and the 18" (450mm) size has a double plumb vial. These vials are adjustable to a positive setting and are housed in a satin chrome finished brass tube with a friction-fit closing cover to prevent breakage. The base of the levels features an involute groove running the length of the base, which provides a reliable seat for round work such as pipes or shafting. With the cross test vial, it is possible to simultaneously level in both directions. This prevents inaccuracies in the main vial reading caused by canting the level sidewise on round work. The 6" through 18" (150-450mm) main level vials have graduations that are approximately 80-90 seconds or . 005" per foot (0. 42mm per meter). There are five, six, or seven lines on each side of the bubble, depending on the base length.