Note that although tuning pins may appear square to the casual observer, they are actually tapered slightly toward the tip, with flattened sides, as shown in this illustration. The area enclosed by the red line is the flattened surface, which appears on all four sides (the portion below the blue line is hidden in the pinblock). A truly square pin, which is very, very rare, would appear with four perfectly even sides and no taper. Compare this illustration to the pins on your piano.