What is a chord of wood?

The truss rod can be adjusted to compensate for expansion or contraction in the neck wood due to changes in humidity or temperature, or to compensate for changes in the tension of the strings (the thicker the guitar string, the higher its tension when tuned to concert pitch). Also, different players often prefer different string heights (called the guitar's "action") depending on the style of music they play. For instance, a rock guitarist will often prefer a lower action than a jazz guitarist. Acoustic guitars also usually have higher actions than electric guitars.