A variety of mechanical vibrato systems for guitar have been developed since the 1930s. They are used to add vibrato to the sound by changing the tension of the strings, typically at the bridge or tailpiece of an electric guitar using a controlling lever (often referred to as a whammy bar, vibrato arm/bar, or tremolo arm/bar). The lever enables the player to quickly vary the tension and sometimes the length of the strings temporarily, changing the pitch to create a vibrato, portamento or pitch bend effect.