If there is no page to be moved from Q (as Q has reached its minimal size) then the values in P and Q may be merged, adding the middle item from the ancestor page of P and Q and tehn entirely dispose of page Q
If there is no page to be moved from Q (as Q has reached its minimal size) then the values in P and Q may be merged, adding the middle item from the ancestor page of P and Q and tehn entirely dispose of page Q
This is the inverse to page splitting.
Once again removal of the middle value in the ancestor page may cause the size to drop below n and balancing or merging will be required at the next level.
This is the only way a B tree may shrink in height.