In some piano music, you may have three different levels, as it were, e.g.
> a melodic line in the treble, played by the right hand, a deep bass
> consisting of long notes in the left hand, and in between chords filling in
> the time between the bass notes, also played by the left hand. In such a
> situation, which would likely involve large jumps of the left hand that may
> even reach into the realm of the treble clef, the score is easier to read
> if you distribute the notes played by the left hand on two staves and have
> the line of the right hand in the third (uppermost) staff.
And sometimes four can be used, as in Rachmaninoff's Prelude in C# minor
where the number of staves to a system fluctuates between two and four: