A fret setting caul helps to distribute the force across the entire fret while hammering it into the board, and reduces the likelihood of marring the fret board with a miss from the hammer. It's made from scrap wood, and custom built for the radius that it will be used for. A set or a single caul can easily be made from the video and the directions on the website.
![](https://s2.save4k.ru/pic/Ts9bUzOtsIs/mqdefault.jpg)