Is it the cadet branches?

At 39 years of age, Kiko, known as Princess Akishino, faced a problem common to many wives in history.

Her husband wanted a male heir, and no heir existed.

Her husband wanted an heir because his country needed an heir.

She’d given birth to two daughters. But that wasn’t enough.

The dynasty is strictly patrilineal.

So what would have happened if, at approaching age 40, she had not finally born a son?

The only heir.

Well, for some time, the dynasty had cadet branches.

Three Emperors have been produced this way.