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 chosen to sit underneath it. Change the one underneath and the percentage changes, without anything about the product changing at all.

All three percentages in that picture are arithmetically true. That is the part worth sitting with: nobody has to lie to show you −31% instead of −4%. They only have to choose which price to subtract from.

What the rules actually require

In the United States there is no federal rule fixing a window. The FTC’s guides against deceptive pricing say a former price has to be a bona fide one — a price the item was actually offered at, openly and for a reasonably substantial period, not a figure set up to be struck through.

That is a standard about honesty rather than a number you can check at a glance, which is why the most useful reference on a U.S. product page is often not the crossed-out list price at all.

• The typical price shown by Amazon.com on many listings is the median of what customers actually paid over the previous 90 days. It is far harder to inflate than a list price, because it is made of transactions rather than of an intention.
• Europe is stricter on paper: the Omnibus directive requires any announced reduction to state the lowest price applied in the previous 30 days. It does not stop a seller raising a price in September — it stops them pretending the November price is a saving on it.
• The United Kingdom sits in between: no fixed window, but guidance that a reference price must be one the retailer genuinely charged, and recently.

Three different rules, one consequence for you: 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 one, and the reason the European 30-day rule exists at all. In a market without that rule, a list 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 an extra controller. That can be genuinely good value — but it is not a discount on the console, and the percentage shown is not the one you would get buying it alone.
• The model made for the season. Manufacturers build variants with their own model codes for this window, sold alongside the normal line. Nothing wrong with that in itself, but the specification is not the one reviewed elsewhere, and the list price it is discounted from has no history at all.
• The countdown that resets. A timer is a feeling, not an offer. If the same deal is still there the next morning with a fresh countdown, the scarcity was a design choice.

What we do before we publish

Four checks, and they are the same every day of the year — Black Friday only makes them matter more.

• Use the reference the store itself shows, and say which one it is. We quote the typical price when the page gives it, and we write that we are quoting it.
• Check the exact model code, not the family name. A name that stays the same across revisions is the most common way for two different products to look like one.
• Check the condition. Renewed and open-box can be real bargains, but they are not new, and a discount measured against the new price is a comparison between 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 rather than a side effect.

This is also why we say when a price was checked. A price is a fact with a timestamp, and without the timestamp it is just 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 a list price: the price on the page, this week, with the date next to it.

On the 27th you will have the only reference that is genuinely yours, and it is 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 the same thing 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.