Countersunk hole is to be placed directly atop the front rail pin of the two most extreme notes (remove all shims and felt). A short bolt is used in the opposing hole as a height-adjustment to set a desired level height. Then, these blocks are left in place while the remaining white keys are leveled in reference to a long straightedge (e.g. WNG key level stick). Use with Sharp Leveling blocks to do the black keys!