A guitar callus is a thick pad of skin that develops on the fingertips of a guitarist's non-strumming hand as a result of repeated fretboard and string manipulation. This hardened region of skin develops over time as a result of the body's defensive defenses responding to repetitive and persistent friction and pressure on a section of skin. Calluses are common among guitar players because they require more control of the string's tension than other instruments do, thus making it important for your fingers to be properly protected. When playing chords or barre chords, your index finger and thumb work together to press down each of the strings simultaneously. As you progress in skill level, you will need to apply less force with each chord, which will allow you to play more notes in a row. The more frequently you play an instrument, the more likely you are to develop calluses on your hands. They will not only protect your fingers but also enhance your technique by giving you greater control over the string's tension. Calluses can be caused by any number of things such as rubberized guitar picks, wood or stone fretboards, or even metal machine heads. The more contact points there are between your skin and some hard object, the more likely you are to develop callouses. If you are struggling with calluses, it is important to avoid picking at them until they heal up completely. Picking at them will only cause you further pain as well as damage to the skin tissue.