Here's a situation we see constantly. A couple build a company together. One of them does most of the fee-earning and ends up a higher-rate taxpayer, drawing dividends taxed at 35.75% in 2026/27. The other works in the business too, answering the phone, doing the books, holding the whole thing together, but takes little or no income. Their personal allowance and their entire basic-rate band sit completely unused, year after year.
Meanwhile the higher earner is paying the top dividend rate on money that, if it landed in the other spouse's hands, would be taxed far more gently. That gap between two tax positions is real money, and using both sets of allowances is one of the most legitimate pieces of planning a family company can do.
The tool is alphabet shares. The trap is the settlements legislation. The pillar guide to alphabet shares runs through the basic scenario, so I won't repeat it here. This article goes deeper on the bit that actually decides whether the planning is safe, the law around gifts between spouses, and the famous case that settled it.
Income tax is personal. Each spouse has their own personal allowance (£12,570 in 2026/27), their own dividend allowance (£500), and their own basic-rate band before dividends start being taxed at the higher rate. If only one person in the household is drawing dividends, the other's bands are wasted.