is hit within that window, the player receives the same number of points. Selected special segments of the song will have glowing notes outlined by stars: successfully hitting all notes in this series will fill the "Star Power Meter". The Star Power Meter can also be filled by using the whammy bar during sustained notes within these segments. Once the Star Power Meter is at least half full, the player can activate "Star Power" by pressing the select button or momentarily lifting the guitar into a vertical position. When Star Power is activated, the scoring multiplier is doubled until Star Power is depleted. The Rock Meter also increases more dramatically when Star Power is activated, making it easier for the player to make the Rock Meter stay at a high level. Thus, Star Power can be used strategically to play difficult sections of a song that otherwise might cause the player to fail. In the earlier entries of the series (up until Guitar Hero: Aerosmith), activating Star Power meant that players could not accrue more Star Power until the Star Power meter was fully drained and the effect ended. Starting with Guitar Hero: World Tour, more Star Power can be collected even if the effect is active by completing more Star Power phrases, extending the Star Power's duration by doing so. When playing in cooperative play (with a bassist/rhythm guitarist in Guitar Hero II through Guitar Hero: Aerosmith or as a band in Guitar Hero: World Tour), Star Power is shared between all the players and activation of Star Power is dependent on all players simultaneously activating it. Notes can be a single note, or composed of two to five notes that make a chord. Both single notes and chords can also be sustained, indicated by a colored line following the note marker; the player can hold the sustained note(s) keys down for the entire length for additional points. During a sustained note, a player may use the whammy bar on the guitar to alter the tone of the note. Also, regardless of whether sustains are hit early or late, if the fret is held for the full duration of the hold, the game will always award the same amount of score increase for the note. In addition, the games support virtual implementations of "hammer-ons" and "pull-offs", guitar-playing techniques that are used to successfully play a fast series of notes