click the lines and spaces to play them like an instrument :) you can also press space to change the style of the bass clef symbol :)
the lowest ledger line pictured here is "low C" and the highest is "middle C" ( the yellow lines at the top and bottom ) https://en.wikipedia.org/wiki/Clef#/media/File:Clef_Diagram.png https://en.wikipedia.org/wiki/C_(musical_note)#Designation_by_octave
