The sextile aspect in astrology is where two planets are sixty degrees apart. Here the circle is divided by six. It is related very strongly to the major trine aspect.
The sextile aspect is half the angle of the trine aspect. With the trine aspect, the planets are generally separated by three signs in between. Since there are four elements, the planets involved in a trine have the same element, e.g both fire. With the sextile aspect, the planets are generally separated by one sign in between.
Unlike the trine, the sextile planets do not share the same element, but they will have compatible elements. Air and fire are compatible and water and earth compatible. The former more yang and extrovert, the latter more yin and introvert. The sextile aspect is therefore slightly more difficult than the trine, although still advantageous. Whereas the trine gifts come easily and without effort, the sextile involves an ability, but it has to be worked for. This is hinted at by the fact that the number six is made of three times two. Three is the number of harmony and balance but two is a number of division and dynamism. Elements of the psyche are related, and potentially powerful when operating in harmony, yet this gift has to be worked for.