Sometimes replacement parts need to be made from sheet metal. In order to fit properly, the holes for rivets, screws and bolts must be perfectly aligned with those on the rest of the aircraft. The easiest way to align these holes is to back-drill through the structure, but sometimes this isn't possible. Hole finders, sometimes called strap duplicators, allow the builder to accurately drill holes to match an existing hole pattern. The pin on the bottom of the strap is inserted into the old hole, the new piece is slipped between the straps, then the hole is drilled through the bushing on the top strap.