Whenever a string is pressed to a fretboard, the tension of the string is increased. This causes the fretted note to be slightly sharp compared to the open string note. This must be compensated for. To compensate for this sharpness, the distance from the nut to the bridge saddle is made slightly longer than the stated scale length for the instrument. This lowers the pitch of the fretted notes slightly. The amount of compensation needed depends on how far the string must be pressed to the fingerboard (action), the mass/thickness/gauge of the string, the tension (tightness) of the string, and the string length. When you look at the bridge of most instruments, you see evidence of compensation, for example; when you see the guitar’s saddle at an angle to the strings rather than perpendicular to them. On a mandolin, some guitars and banjos, and electric guitars in particular, the saddle is carved or adjusted so each string bends over it at a different distance from the nut. The exact amount is usually determined by comparing the pitch of the note fretted at the 12th fret to the pitch of the harmonic at the 12th fret. When the two match, you have found the compensated position of the bridge saddle for that string.