<html> Rogerio, When the butt felt is the correct thickness, the center line of the jack should be perpendicular to a tangent drawn on the butt skin at the jack contact point. In other words, select the butt felt that leaves the jack pointing straight at the curved surface of the hammer butt skin. Manufacturers sometimes tweak this to give a stronger or weaker letoff resistance, but the perpendicular setting is the "conventional" design setup. This is a very badly neglected area of service in old pianos, I guess because of the cost of replacing the butt felt and butt skin.  When these are replaced in an old action, the performance can be very good. Don Mannino RPT At 07:20 PM 12/20/00 -0200, you wrote: <blockquote type=cite class=cite cite>To the list   There are very good articles and discussions about the position of the jack under the knuckle in a grand, but there is no discussion about the position of the jack under the hammer butt in a upright piano. I would like to know the correct thickness of the butt felt that must be applied in a piano. I think that "medium" and "thin" isn't good.  I thanks to all Rogerio Cunha IC Member of the Piano Technicians Guild Rio de Janeiro - Brazil</blockquote></html>