One of the fine points in making a neck is that the fretboard should have a slight dip along its length. This is called "relief" and makes it possible to set the strings a bit lower with respect to the top of the frets. The result is an instrument that plays more easily and accurately. The dial indicator on this shop-built device shows relief of a little over three thousandths of an inch in a tenor ukulele neck. Once the instrument is completed, the string tension will pull a bit more curvature into the neck and it should be just about right.
