Hi Duoplicity,
The second line in the staff with treble clef marks G key in the middle octave ( this is G4).
The fourth line in the staff with bass cless marks F key and this is one octave down from middle octave.
Octaves are counted as sectors from one C to another C. So that every octave (sector on the keyboard) contains C,D,E,F,G,A and B. Then the next C starts another octave.
GL
Jaak
P.S.
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