There is a magnet at the bottom,which can be adsorbed on the top or bottom of iron items,making the measurement more accurate and convenient,Perfect for woodworking projects,furniture installation. The T Bubble is precise,and provides for front-to-back or side-to-side leveling,with 2 optional screw holes for permanent mounting or apply adhesive for mounting. The String Line is,lightweight,and resistant to wear.With moulded hooks to enable the to be hung from a builder line.This bubble is accurate,and lightweight,it can be used for leveling the position of telescopes,tripods,cameras. The spirits bubble sets contains 4 different types of levels: Magnetic pocket,t,string line & circulars. This bubble comes with a compact box,which can easily store these small tools and is very convenient to carry.