Why do prices end in .99?
People read prices left to right and stop paying attention early, so £9.99 lands in the mind as 'nine something' rather than 'basically ten'.
Simple intuition
The plain reason, in everyday words
Reading a price is not a careful calculation; it is a glance. And a glance goes left to right and stops as soon as it has enough to place the item roughly. Faced with £9.99, the first thing your eye lands on is the 9, and the number gets filed near nine rather than near ten — even though it is one penny away from ten. The single penny does almost nothing to what you pay and a surprising amount to how the price feels. There is a second, quieter signal too: a price ending in .99 has come to mean 'this is a keen price', while a round £10.00 reads as considered, or premium. So retailers use the ending as a message about what kind of price it is, not only as a trick about the first digit.
It works because people are stupid or not paying attention.
It works because reading a price is a fast heuristic judgement, which is usually the correct thing to do. The effect exploits an efficient shortcut rather than a failure of intelligence.
It always increases sales.
Effect sizes vary widely and several studies find reversals. In premium categories the ending signals discounting and can reduce willingness to pay, which is why luxury retailers avoid it.
It was invented to stop cashiers stealing.
That story is widely repeated but weakly evidenced. Odd pricing appears in newspaper advertising as a competitive device before it can be explained by cash register discipline.
Charm pricing is just about the last digit.
The catalogue experiments where $39 outsold $34 show the price level itself is being read as a signal, which no purely digit-based account explains.
It is a good calibration exercise: an effect everyone has heard of, which is real, smaller than advertised, sometimes reversed, and attached to an origin story that does not survive checking. Learning to hold those four facts at once is more useful than the pricing lesson. It also shows how much of consumer behaviour is fast pattern-matching rather than calculation, which is the foundation of most of behavioural economics.
Who worked it out
Odd pricing appears in American newspaper advertising in the late nineteenth century, where a price just under a round figure was a visible competitive claim.
What problem forced it
The cash-register theft explanation became the standard telling in the twentieth century despite thin contemporary evidence, and it persists largely because it is a satisfying story.
How it changed since
Systematic testing began in the mid-twentieth century and accelerated with retail field experiments in the 1990s, which established that the effect is real, category-dependent and entangled with what the price ending signals about quality.
Why anchoring changes what you will pay
The first number you see shapes every judgement after it, and retailers arrange which number that is.
How decoy options steer choices
A third option nobody buys can change which of the other two people pick — the same family of effect.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.