Also called: survivor's penalty, single-filer penalty
After a spouse dies, the survivor files as Single with nearly the same income. Brackets and the standard deduction are roughly halved and Medicare thresholds drop, so taxes and premiums rise.
A joint return is still allowed for the year of death. From the following year the survivor is Single unless there is a dependent child. Household income usually falls by less than half: the smaller Social Security benefit stops, but RMDs, pensions with survivor options and investment income continue.
The thresholds for IRMAA, the net investment income tax and the taxation of Social Security are all lower for single filers. This is one reason to compare Roth conversions made while both spouses are alive.