# Why invoice numbers can't change

Invoice and credit note numbers run in order with no gaps, and why a number can never be reused or edited.

In the UK, invoice numbers should run in order without gaps, so your accountant and HMRC can see nothing's missing. Sustainable ERP makes sure of it:

- A number is given only when you issue the invoice, not while it's a draft. Deleting a draft never leaves a gap.
- Numbers are never reused, even if something goes wrong.
- An issued invoice can't be edited or deleted. Mistakes are put right with a credit note.

Credit notes have their own sequence, with the same rules.

You can change the **Prefix** (for example INV-) and the number of digits in **Settings → Document numbering**. The next number can only move forward.