Most importantly, the T-Rule operates at a mechanical pitch of 0.5. Accurately drawing lines is a true blessing. With age, eyesight deteriorates. The T-rule allows you to determine exactly where the lead should be. You insert the lead of the pencil into the hole that corresponds to the size you want. Move the tool and you'll get straight lines, not shaky lines, right where they should be. I then use double sided tape to hold everything in place while I screw or nail the object in place.