Distance between 2 notes on the bars with F and G clef is?

[lionlion]

For instance:
2 music bars. Upper is G clef and lower is a F clef. There are 2 notes. G clef the G (where the clef hits the line) and a F (where the F clef hits the line)

What is the distance between these 2 notes?

My idea is, correct me if I am wrong. There is an central C between the 2 bars, which both bars share in common. so I would start counting from G clef down to F clef: I count 9 steps (including G).
That is G, F,E,D,C,B,A,G,F (Central C is in bold)
But so sometimes I see people write many note lines between the 2 bars. So actually how many lines are there between the 2 bars I wonder?

[Nobdi]

You are correct with that there is a "c" on an additional line between the 2 note systems (as shown here):

https://qph.fs.quoracdn.net/main-qimg-3 ... 9e5bdc2383

The "many additional lines" are only used when you just use 1 of the systems (G clef or F clef or whateverclef) and only have a few notes above or below the used system. It would just be overkill (and make the whole thing harder to read ) to add a second (or 3rd) note system just for a few notes.
In your example the 1st line below your f clef system would than be "c" and the lines below that would kind of be a short bit of your G clef system below.

[lionlion]

Thank you.

This would mean that some teachers are actually teaching things wrong: 2 clefs and more then 1 line between the upper and lower bar,

[Nobdi]

Technically I would say it is wrong but from a practicle standpoint it might just be that it is a notation for more than one instrument and the author/transcriber wants to make it easier to read for people who are only used to 1 single note system at a time.
If you ask me - allowed is all that works

[jancivil]

"actually teaching things wrong"... we're seeing 'bars' for staves and 'music lines' for ledger lines, but one is here to say what's right or wrong in the wide world of notation.

I was a classical guitarist, majored in it, toured with it. It's rare to see another clef in the literature.
That's the convention, that's the expectation. Even as it's written an octave higher than it sounds (with a little 8 at the curlicue at the bottom, "8ve bassa" it's called, same as the double bass, the bass flute, and generally contrabass instruments), it's ledger lines all the time, below and above the staff; except the above the staff stuff might be indicated by "8va" with a bracket over the octave above bits, if it goes on for a while.
Same with notes below the bass clef, it can be '8ve bassa' brackets or ledger lines.
It's not a piano part. The grand staff is not used for monophonic instruments either. It's not used for anything but keyboards as far as individual performers go.

additionally in four part writing in grand staff, there are occasions where ledger lines are employed, to be clear about what voice; and the individuals executing parts are reading their part. BTW there's a third clef (movable!) in common usage designed to mitigate ledger lines somewhat... the 8ve bassa convention is there to avoid a lot of clef changes, in favor of ledger lines. I'm not sure why it [eg., guitar] isn't tenor clef, but it just never was (I led the ensemble because I was the reader basically and pretty sure no one read tenor (or alto, baritone) clef and that's what we have, convention.

[fmr]

Thank you.

This would mean that some teachers are actually teaching things wrong: 2 clefs and more then 1 line between the upper and lower bar,

No, it isn't wrong. When you are writing music where there are horizontal (melodic) movement (for example, for a choir, or an orchestral transcription, or simply polyphonic four part writing) is what matters, you usually keep the first two voices (in choir or four part writing) in the treble clef, and the two lower voices in the bass clef.

If you are making an corchestral transcription, usually violins are transcribed in the treble clef, while cellos and basses are transcribed in the bass clef. However, not only can the violins play below central C, cellos may (and many times do) play above central C.

OTOH, violas may be transcribed in treble or in bass, depending on the predominant register used.

Even piano music, if written polyphonically, may follow this convention (having upper parts going below central C, and lower parts going above). Many music is not written in "chords".

[lionlion]

Thank you for the extra explanations.

The last few answers are way above my head at this moment. Just learning music theory for creating background melodies in the future. No idea if I will use it professionally.