In Saju, characters interact. Stem combinations: Gap+Gi (Earth), Eul+Gyeong (Metal), Byeong+Sin (Water), Jeong+Im (Wood), Mu+Gye (Fire); stem clashes: Gap–Gyeong, Eul–Sin, Byeong–Im, Jeong–Gye.
Branch Six Harmonies: Rat+Ox, Tiger+Pig, Rabbit+Dog, Dragon+Rooster, Snake+Monkey, Horse+Goat. Three Harmonies: Monkey-Rat-Dragon (Water), Pig-Rabbit-Goat (Wood), Tiger-Horse-Dog (Fire), Snake-Rooster-Ox (Metal). Clashes are the opposite pairs: Rat–Horse, Ox–Goat, Tiger–Monkey, Rabbit–Rooster, Dragon–Dog, Snake–Pig.
A clash is not "bad luck" by itself — traditionally it means movement and change, read with the whole chart.
Frequently asked questions
Q. Is a clash in my chart bad?
A single clash does not decide good or bad. Traditionally it is read as change and movement, together with the balance of the whole chart.
Q. Is it the same clash as in zodiac-animal compatibility?
It uses the same branch clash table (Rat–Horse, Ox–Goat, Tiger–Monkey, Rabbit–Rooster, Dragon–Dog, Snake–Pig). But looking only at the year animal is less precise than reading all eight characters.
Q. What is a half combination?
When only two of the three Three-Harmony branches are present, including the middle one (Rat, Rabbit, Horse or Rooster). Example: Monkey and Rat alone form a Monkey-Rat half combination (Water).
Related: What are the Twelve Life Stages in Saju? · How does Saju compatibility work?