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The purpose of the commorientes rule is to determine who should inherit an estate, i.e., it answers the question of succession. The rule and the presumptions it implies do not apply for inheritance tax purposes.
Where the commorientes rule does apply between two individuals, for the calculation of inheritance tax in each of their estates, the two deceased persons presumably died at the same instant. This prevents double taxation of the elder’s estate.
For example, if a mother and her son die at the same time, and the mother leaves her entire estate to her son, inheritance tax will not be payable on the mother’s estate and then the combined value of the mother’s and the son’s estate. Instead, inheritance tax is separately charged on both estates. This is helpful for tax and in terms of the administrative burden for the probate process.
Elisabeth's closing comment
This type of case is uncommon, with it being unusual to have to rely on the commorientes rule to determine the beneficiaries of a couple’s estate.
However, this highlights the importance of having a written Will to ensure that your wishes are properly recorded and to avoid an unpredictable and potentially unwanted distribution of your estate upon your passing.