Harmonic horoscopes are based on principles of resonance, like overtones, which are present in every horoscope. The whole zodiac (360°) is taken as the basic tone, representing the number one (1). By using a higher vibration, we can “cause” the circle to oscillate more quickly, so to speak, and investigate which planets work together in this particular pattern. For example, the fourth harmonic would involve all those planets which share square aspects (90° - division of the circle by four). In the harmonic chart, these planets form conjunctions. The number corresponding to each “vibration” influences the interpretation.The aspects imply favourable, unfavourable or ambivalent relationships between the planets. Although the interpretation depends greatly on the nature of the planets involved, this view largely derives from the traditional symbolism of the numbers 1,2,3 and 4: When the circle is divided by the number one, the result is again 360° or 0° the distance which defines a conjunction. Division by 2 gives us 180°
The number one symbolises the original oneness of all being, and consequently two planets in conjunction become a unified whole. The number two is the division of this one-ness into polarities, for example male and female, upper and lower, yin and yang, etc., leading to a certain tension of opposites. The number three symbolizes a creative striving – thesis and antithesis leading to a synthesis. Usually, planets in trine are in the same element. Accordingly, they support and complement each other. The number four is matter made manifest, law fulfilled, that which has become. The corresponding square is generally considered difficult, each side obstructing the other, unable to give way.
Source: astro.com