This is the original version of Hass notation. It uses a five-line, pitch-proportional chromatic staff whose lines are spaced three semitones (a minor third) apart. Three noteheads shapes are used: an oval on lines, and downward and upward triangles in the spaces. This regularly alternating pattern of three noteheads corresponds directly to the three rows of a chromatic button accordion, which has an isomorphic layout. The staff does not cycle at the octave (MNP criterion 9). Hass later created a version that does.
The traditional treble and bass clefs are replaced by an E and an O clef, respectively. As in traditional notation, each of these clefs gives a different set of note names to the staff lines. A note on the lower staff is an octave and a major sixth lower than the note at the same position on the upper staff.
An unusual property of the system is that the majority of the positions on each staff are the same as in traditional notation (as long as one applies the necessary sharps and flats in traditional notation). The stem is horizontally centered on the notehead, rather than placed at the left or right as in traditional notation. Rhythmic notation is traditional.
Pitch nomenclature uses traditional German pitch names for the naturals (with B for the pitch a whole step below C, and H for a half-step below C), and the new names O, S, V, and I for the traditional C#, D#, F#, and G#.
Hass, who died in September 2015, stated in 2009 that he was not sure exactly when he invented this notation system but believed it was 1974.
See also the Hass Notation Website (Denmark), and the three-line version of Hass Notation on our site.