A transitional serial interest (TSI) can arise in three ways:
As an IIP that arises between 22 March 2006 and 5 October 2008 (inclusive), and which follows a previous IIP in effect immediately before 22 March 2006 (called "the previous interest") (sections 49B(a) and 49C, IHTA 1984).
On the death of the holder of the previous interest, if he died after 5 October 2008 and the new life tenant is his spouse or civil partner (sections 49B(a) and 49D, IHTA 1984).
To the extent that the trust property is a life insurance policy, if there has been an unbroken chain of IIPs ending on death, beginning with the death of either:
the holder of the previous interest (provided that he died after 5 October 2008); or
a person with a TSI under section 49C of IHTA 1984 (sections 49B(b) and 49E, IHTA 1984).