What is a class gift?

A "class" is a bunch of people that can be grouped together based on some shared factor or feature. Imagine if Amy had 100 children and you wanted to convey each of them an equal share in Blackacre. You could convey a share to each of them, one by one, by name. Alternatively, you could simply convey Blackacre "to Amy's children."

The benefit of this is it is super efficient. Not only did you save time not having to name everyone out, but if Amy manages to have another child, you don't need to make another conveyance since your class gift was pretty broad. In other words, "to Amy's children" means to all of Amy's children, currently born, or later. The fact that this is "open ended" is why such a class gift is said to be subject to open. Today it's 100 slices of pie, tomorrow it's 101, and by the time Amy dies it may be more. Note that under this conveyance, Amy could add to the class by adopting. If you wanted to prevent that, you'd likely need to say something like "to Amy's naturally born children."

But what if Amy doesn't yet have any children when you convey "to Amy's children"? Then the members of the class (her children) are unascertained, which means the class gift is contingent on them being born.