4" diamond coated hook and knife file, md grit side has double grooves. When sharpening the hook, always start with the md grit side. The side with two grooves. Hold the hook flat in the bottom of the groove, and pull from one end to the other a couple times. Then flip over the file and do same thing once on the fine grit side. Test to see if hook is sharp enough on your fingernail, if it sticks easily it is, if not go back to the md grit side and pull the hook along the length of the groove at an angle - lean the hook into the side of the groove, one pull on the left side, one on the right, and the last right down the middle. Then back to the fine grit side and test again.
Diamond coated flat section for sharpening knife blade
Double Groove MD grit to increase performance and longevity
Rounded ends to reduce harm to pockets and increase comfort