Cutting the rear rabbet
The rearmost ¾” [19mm] of the keyboard blank must be thinned down to 3/16” [4.7mm] in thickness. Making this area thinner cuts down on the amount of wood the back rail pins have to go through which helps to eliminate excess friction and noise. Find the Line TU which is ¾” [19mm] from the rear edge. Cut a rabbet 5/16” [7.9mm] deep from that line out to the rear edge of the blank. What remains should be 3/16" [4.7mm] thick. It should look like this.
