Your tax code is used by your employer or pension provider to work out how much income tax to take from your pay or pension. You can find out more about tax codes on the GOV.UK web site. A tax code can change for a number of reasons, depending on what is included in your code number and your own circumstances.
For those who only receive the basic personal allowance, which in 2017/18 was £11,500, your tax code for that year was 1150L (1150 refers to the standard tax-free Personal Allowance figure of £11,500 divided by 10). The code will usually only change when the Chancellor announces changes to the level of the allowances as part of the autumn Budget.
If, however, your tax code includes deductions to collect tax on those parts of your income which have not already been taxed, you may see more regular changes to your tax code as these amounts change. This is so that where possible, the tax deducted by the end of the year is correct. Changes in tax code numbers will therefore reflect changes in your personal circumstances.