Page "Rule against perpetuities" Paragraph 22

In the above example, T's children are a valid measuring life.

T's children are a class, so the class must be closed at the time of the instrument for T ’ s children to be valid measuring lives.

Here, the class that consists of T's children would be closed at the time of the instrument as it is impossible for T to have any children after T dies.

So any interest which must vest within 21 years after T's children die is valid.

The class that consists of T's grandchildren is NOT a valid measuring life as T's children are free to reproduce after T dies, meaning the class is not closed at the time of the instrument.

Obviously, the same goes for T's great-grandchildren for the same reason.