In theory a strings halfway point should be at fret 12 and, when fretted, should raise the pitch by an octave. However, fretting the string increases the tension and therefore causes a small increase in pitch. To counter this increase in pitch the distance from the nut to the saddle is increased (making the string longer). This extra distance is called the compensation. Various factors affect how much compensation is needed: the action, gauge, tension and length of the string all need to be taken into account. This explains why the saddles on guitars are staggered. The compensation can be found by adjusting the saddle until the pitch of the fret 12 note matches the harmonic at fret 12.