This form has to be used where someone has died after 9 October 2007 and his or her spouse or civil partner died before them without using up all of the nil rate band available, and it is intended to transfer the unused amount to the more recently deceased's estate.
HMRC have also recently published details of nil rate bands going back to 1914, since these will be required to calculate the amount to be transferred under the new rules proposed in the Pre-Budget Report.