There are several legal,
Ideally:
Men kill women, and both sides can support their own vitality.
And:
One side's life is weak, and the other side makes up for the life of the weak side;
One side is the strong in life, and the other side lacks the strong elements of the strong side;
One side is short of money, and the other side is rich.
……
There are many situations.
To put it simply, one side is particularly poor in some aspects, and the other side can't make up for this defect, but it will add fuel to the fire.
Few girls born with eight characters teach their children or boys born with eight characters honor their wives and help their children. Generally speaking, eight characters are flawed, but whether these defects can be made up by marriage is the basic idea of eight-character marriage.
If you really want to know, you can watch Three Sessions of Life and learn from the beginning. You don't have basic knowledge. Your question is so general that I can only give a general answer.