What are they?

Tax codes are made up of an array of letters and numbers, each of which indicates something about the persons’ tax liability. 

The number 1257 is the most common as it shows how much personal allowance one has in the tax year before their income becomes liable. 

Currently the personal allowance is £12,570, which is why many employees will have the 1257 number on their tax code. 

The letters on one’s tax code indicates any further allowances one can receive and the tax rate they should be charged. 

Which? recommended that people who believe their tax code is wrong should contact HMRC, as the responsibility to ensure a tax code is correct is on the taxpayer themselves.