This is the thickness of the drive links (the tooth-like underside of the chain that fits in the guide bar). The chain gauge is measured in inches and will usually be in the manual or on the chainsaw itself. As it is difficult to measure accurately with a tape measure, you may need a vernier caliper to get this measurement. There are four different gauge sizes, which are .043, 0.50, .058, .063, with the lower sizes being the most common. If you select the wrong size, it may be too thick to fit your chainsaw or too loose to gain proper traction.