structural:expansion
Historical background

The rule has its origin in the Duke of Norfolk's Case of 1682. That case concerned Henry, 22nd Earl of Arundel, who had tried to create a shifting executory limitation so that some of his property would pass to his eldest son (who was mentally deficient) and then to his second son, and other property would pass to his second son, but then to his fourth son. The estate plan also included provisions for shifting property many generations later if certain conditions should occur.