The marriage tax allowance allows you to transfer some of your personal allowance to your spouse or civil partner if they earn more than you.
If your claim is successful, it will lower the higher earner's tax bill for the tax year, but you can also backdate your claim if eligible.
One of you needs to earn less than the £12,570 personal allowance between 6 April 2023 and 5 April 2024. To get the full benefit, the non-taxpayer actually needs to earn £11,310 or less.
The other partner needs to be a basic 20% rate taxpayer. This means you'd normally need to earn less than £50,270.