====== Resource ======
  * Free learning resource: https://www.studybass.com/
  
notes:
  * bass chord is hard to hear clearly, so sequence of note play is used
  * rhythm: (节奏) timing of notes
  * harmony:(和声) several notes played together
    * chords: group of notes at the same time
    * chord progression: sequence of chord
    * scale: set of notes; major, minor, ...
      * key: a scale around a key
  * melody: (旋律)