The Marriage Allowance came in from 6th April 2015, ie for the 2015/16 Tax Year.
It applies to married couples and civil partnerships where one spouse or partner has income of less than the Personal Allowance of £12,500, and the other partner is a basic rate taxpayer.
The effect of the Marriage Allowance is that the non-taxpayer can transfer £1,250 of their Personal Allowance to their spouse or partner. If the partner can utilise all of the £1,250 against their income they will receive a tax saving of £250. These figures are for the 2019/20 Tax Year.
Marriage Allowance can only be claimed by people who were born after 6th April 1935, where one spouse or partner was born before 6th April 1935, they can claim Married Couple's Allowance.
People who complete Self Assessment Tax Returns each year apply for the Allowance on their Tax Returns. People who do not complete Self Assessment Tax Returns need to register with HMRC How to Apply for Marriage Allowance .