Registers are strips of wood with rectangular slots sized to exactly fit the dimensions of the jacks that must pass through them. The slots are spaced apart according to the spacing of the rear of the instrument's keys. The size of the slots, and the space between them, is critical to the functioning of the harpsichord. Not to be secretive, but the process of accurately producing these registers is far beyond what I can describe in this limited space. I devote 67 pages and 1 1/2 hours of video to the process in my book, the Harpsichord Project - a testament to the importance of these assemblies. In the photo, you can see two jacks sitting in the front register. If the registers were made with the necessary attention to precision, the bottom of the jacks will sit directly in the center of the keys that they rest on.