If you try bending a short piece of piano wire or guitar string slightly with your fingers, you can feel the wire's resistance to bending. In a vibrating string, that resistance adds to the effect of string tension in returning a given part of the string toward its rest position. The result is a frequency of vibration higher than the theoretical frequency. And because the wire's resistance to bending increases as its length decreases, its effect is greater in higher harmonics than in lower.