Planets that are 120 degrees apart from each other are said to be in a trine. When you divide a circle by three, you get a shape that looks like a triangle, hence the term trine. Harmony and flow are characteristics of the trine.
The most common form of trine is between planets that are in the same element:
1) Water signs are Cancer, Scorpio, and Pisces
2) Earth signs are Taurus, Virgo, Capricorn
3) Fire signs are Aries, Leo, and Sagittarius
4) Air signs are Gemini, Libra, and Aquarius.
A relationship between the Sun in Pisces and the Moon in Cancer would be called a trine. The basic vitality of the life force-giving Sun would be in harmony with the emotions of the Moon. Astrologers like to be specific about the exactness of what is called a trine, so they look to see how close two planets are to 120 degrees apart before they call it a trine.