A discount is not a number about the product. It is a number about two prices

The number you are shown is a subtraction

A percentage off is not a property of the product. It is the distance between two prices: the one you pay and another one placed underneath it. Change the one underneath and the percentage changes, while nothing about the product changes at all.

All three percentages in that picture are arithmetically true, and that is the part worth sitting with. Nobody has to lie to show you −31% rather than −4%. They only have to choose which price to subtract from.

What the rules require here

There is no fixed window in the United Kingdom. What there is instead is guidance with a clear principle behind it: a reference price — the "was" — should be one the retailer genuinely charged, openly, and recently enough for the comparison to mean something. A figure that sat on a page while nobody bought at it is not a reference.

That puts the work on the wording rather than on a number you can verify at a glance.

• Read what the crossed-out price is called. "RRP", "was", "typical price" and "reference price" are four different claims, and only some of them are about what this retailer actually charged.
• The European Union is stricter on paper: since the Omnibus directive, any announced reduction has to state the lowest price applied over the previous 30 days. It does not stop a seller raising a price in September — it stops the November figure being presented as a saving on it.
• The United States has no window either, but a former price is supposed to be bona fide: actually offered, openly, for a reasonably substantial period. On Amazon.com the more useful number is often the typical price, the median of what customers actually paid over 90 days.

Three different regimes, one consequence: read the words next to the crossed-out number before you read the number.

The four traps of the season

• The price that climbs before it falls. The oldest trick, and the whole reason the European 30-day rule exists. Without a window, a price raised in October makes a November price look like a win.
• The bundle instead of the product. Consoles are the clearest case: the console itself rarely moves, so what goes on offer is a bundle with a game and a second controller. That can be genuinely good value, but it is not a discount on the console, and the percentage is not the one you would see buying it on its own.
• The model built for the season. Manufacturers make variants with their own codes for this window, sold next to the normal range. Not wrong in itself — but the specification is not the one the reviews tested, and the list price it is reduced from has no history behind it.
• The countdown that resets. A timer is a feeling, not an offer. If the same deal is there the next morning with a fresh countdown, the scarcity was a design decision.

What we do before we publish

Four checks, and they are the same every day of the year. Black Friday only raises the stakes.

• Use the reference the retailer shows, and say which one it is. If the page gives a "was" price we say that is what we are quoting, rather than letting it pass as a market price.
• Check the exact model code, not the family name. A name kept across revisions is the commonest way for two different products to look like one.
• Check the condition. Refurbished and open-box can be real bargains, but they are not new, and a discount measured against the new price compares two different things.
• Drop what does not survive. On a day like Black Friday that means publishing fewer deals than everyone else, which is the point and not a side effect.

It is also why we say when a price was checked. A price is a fact with a timestamp; without the timestamp it is a claim.

Country by country

The one thing worth doing this week

Write down what the thing you want costs today. Not a screenshot of a review, not an RRP: the price on the page, this week, with the date beside it.

On the 27th you will hold the only reference that is genuinely yours, and the one nobody can adjust. Every rule above exists to give you an approximation of that number — and you can simply have the number.

It is what our price archive does for the deals we publish, except you can do it in ten seconds for the one product you actually care about.