He proposed a sort of staff notation, upon which all the voices could be represented at once. The following illustration represents his staff and his diaphony, or harmony: [Illustration: POLYPHONIC NOTATION OF HUCBALD.] The initial letters, T and S, at the beginning of the lines in the preceding staff indicate the place of the steps (tones) and half steps (semitones). [Music illustration: DECIPHERING OF ABOVE. Sit glo-ri-a Do-mi-ni in sae-cu-la lae-ta-bi-tur Do-mi-nus in o-pe-ri-bus su-is.] M. Fetis gives a two-voice parallelism in fifths, which is progressively enlarged to three voices by adding an octave to the lower voice; and then to four by doubling the original upper voice in the octave above. Thus: [Music illustration: Tu pa-tris sem-pi-ter-nus es fi-li-us.] In addition to mechanical progressions of parallel motion in this way, Hucbald in another place gives an account of a so-called "roving" organum, in which, while parallel progressions of fourths and fifths still are found, there are also other intervals, while the beginning and the end must be in unison. This form of the harmony of simultaneous sounds has in it much of the character of counterpoint, especially in the restriction that the voices must begin and end in unison. This roving organum, or free organum, was also known as "profane" or "secular" organum, in contradistinction to the "sacred organum" already given, upon the sweetness of which Hucbald greatly prided himself. Fetis has well said that Hucbald must be considered as one of those superior spirits who impress upon their epoch a movement in an art or science. Besides this, he merits particular mention in the history of music because his works are the first since those of Boethius--a period of four centuries--in which the art of music is treated systematically and without obscurity.