A price tag reading $9.99 is one cent cheaper than $10.00. Rationally, that's a rounding error, not a decision-changing fact. Retailers have priced things this way for well over a century anyway, because the arithmetic isn't the part doing the persuading — the way people read the tag is. Most shoppers process a price left to right and weight the leftmost digit most heavily, sometimes finishing their read before they've fully registered the cents at all. $9.99 gets filed mentally closer to "$9" than to "$10," even though anyone who actually does the subtraction gets the same answer either way.
The left digit does most of the work
This is sometimes called the left-digit effect, and it's been tested directly: in controlled studies, shoppers comparing prices that differ only in the cents column — say $2.00 versus $1.99 — perceive the gap between them as larger than the one-cent difference between $2.00 and $2.01, even though both pairs are exactly a cent apart. The leftmost digit changing from 2 to 1 registers as a meaningfully different price bracket; the cents column changing barely registers at all. Retailers didn't need to know the cognitive science to notice the pattern worked — field experiments with mail-order catalogues found items priced at $X.99 sometimes outselling the same item at a slightly lower round price, which is the effect showing up directly in sales figures rather than just in a lab.
This is one tool inside the marketing mix's pricing decision, and it sits alongside — not above — decisions about positioning, product tier and what the price is supposed to signal. A $9.99 tag says "this is a bargain, and we want you doing the arithmetic quickly." A $10.00 tag, chosen deliberately by a brand that could just as easily use $9.99, is often saying something else entirely.
Why some brands price the other way on purpose
Round numbers read as considered rather than squeezed. Luxury and premium brands routinely price at $100 or $500 rather than $99.99 or $499.99, because a price that looks bargain-hunted undercuts the exact positioning the product is trying to hold — a $9,999 watch reads like a discount trying to happen, where $10,000 reads like a number nobody negotiated. The charm-pricing convention only works in contexts where looking cheap is the goal. Where it isn't, the identical psychological trick becomes a liability instead of an asset.
What we're still unsure about
The size of the left-digit effect varies by product category, by how much attention a shopper is actually paying, and by culture — some of the clearest experimental results come from specific retail contexts and don't automatically generalise to every purchase decision, especially high-involvement ones where a buyer is comparing prices carefully rather than skimming. Treating "just-below pricing always outperforms round pricing" as a universal law would be overstating what a genuinely mixed and context-dependent body of research supports.
This sits inside Pricing Strategy, one of seven topics in Marketing, one of four domains in Business, one of seventeen subjects the app can quiz you on.