In ancient Greece, the musicians who played guitar (kithara) while singing were called kitharoedes. Some musicians played guitar but they did not sing . Those musicians were called guitarists. Similarly, an aulode played flute and sang, while an aulete only played the flute (aulos).
The aulites and guitarists used various patterns appropriate to accompany songs, dances and recitations. They called them "laws". Likely, each "law" was a particular rhythm or a sequence of rhythms and sound effects. For example the Pythian law was determined by a specific sequence of rhythmic patterns and a sound effect produced by the aulos . The sound effect was used to represent an imitation of a snake's whistle .
There were several laws, for example the multihead law (πολυκέφαλος νόμος) or the erect law (όρθιος νόμος)
The ancient writers gave great importance to the distinction of consonant and dissonant intervals. Let us use some contemporary musical terms in order to understand this distinction. During those times, intervals like fourth, fifth and eighth were considered as consonant . The rest of them (third, sixth, etc.) were classified as dissonant intervals. However those dissonances were considered to be acceptable inside the tune.
Greek musical scales were descending. Scales limited musical notes and intervals in an ordered group. All scales were structured in tetrachords. Tetrachord, was a system of four sounds (actually four strings of the lyre) covering the interval of a perfect fourth . The Greeks theorists used the tetrachord for their tuning (today we use the interval of the perfect eighth).
According to Aristoxenus, scales were consisted of tetrachords. These tetrachords were either conjunct or disjunct .
a) conjunct: they had one note in common. for example GFEDCBA
b) disjunct: they did not have any common note. For example DCBA GFED'
When we put two conjunct tetrachords together (one on the other), a heptachord is formed. When we put two disjunct tetrachords together (one on the other) an octachord is formed. It is said that Terpander (7th century BC) created a heptachord for first time. According to Nicomachus, Pythagoras (6th century BC) was to create the first octachord.
(pythagorean tuning ,comma )
Genera of tetrachords
“Genus” is a term referring to the intervals' order in a tetrachord. The first and last note of each tetrachord were fixed so to create a perfect fourth. Those notes were called "εστώτες" (fixed notes).
Both internal notes changed their tuning less than a semitone up or down (Microtone). Sometimes they changed their tuning about 1/4 of a whole tone. Those changes were to modulate the tetrachord's genus. The internal notes of the tetrachord were called "κινούμενοι" (movable)
There were three genera of tetrachord: i) diatonic tetrachord. According to Aristoxenus "diatonic genus" was the first that had ever been used and it was the most natural. Everyone was able to sing it, even if he had no musical education. ii) chromatic tetrachord iii) enharmonic tetrachord.