The raised rim around a U.S. coin is not decoration; the Mint forms it before striking so the design wears less and millions of coins can stack more easily

All American coins have a tiny lip of metal along the very outside of the coin which defines its border. We tend not to notice it most of the time, in large part because it usually does not require our attention to do its job. But that thin band of metal is actually one of the more carefully designed aspects of our everyday money. It was created as part of a distinct process of manufacturing, and not as an unintended consequence of a coin being struck.

According to the U.S. Mint report, that raised rim is not some sort of cosmetic addition to the coin. It is built into the coin before being struck, through a process known as "upsetting," wherein a blank is forced through a groove that pushes up the metal into a rim. The rim has two purposes: to protect the design from wear and tear, and to allow coins to stack cleanly. That second point is more significant than it seems in an economy that remains (stubbornly) based on physical change. Coins go through registers, banks, and vending machines every single day, and a coin that doesn't stack or roll predictably not only jams vending machines, but it slows down commerce as well.

Wear and tear, quantified


Coin wear is a real phenomenon that can actually be measured, and has been studied. Economist François R. Velde, from the University of Paris, found in a 2021 study titled “Coin wear: A power law for small shocks,” in Physica A: Statistical Mechanics and its Applications, that coin wear is not random scuffing, but rather the result of a "large number of small shocks" that follow a predictable statistical pattern, allowing the metal loss a coin suffers while in circulation to actually be modelled mathematically. Velde studied this by scanning electron microscopy on a 19th-century gold coin and contended that the curiosity of the scientific community regarding the durability of a coin dates back to the time of Isaac Newton, served at the Royal Mint from 1696, first as Warden and then, from 1699 until his death, as Master, and who had his own concerns about the durability of coins long before the advent of vending machines.

The raised rim is the mechanical solution to the abrasion problem: it is slightly above the flat plane that a coin has. In other words, when the coin skids on a countertop or is rattled around in a jar of coins, the rim gets worn first. It might be a small mechanical fix, but an effective one. That said, the rim is a partial defense, not a guarantee, how well it protects the design also depends heavily on what the coin is made of. Pre-1965 U.S. dimes, quarters, and half dollars were struck in 90% silver, a relatively soft metal that still wears and smooths out with heavy handling, rim or no rim; some of those coins are genuinely hard to read today. Where the rim's benefit shows up most clearly is in the harder copper-nickel clad coins minted since 1965, which tend to hold their detail noticeably longer.

Machines need coins to behave predictably, too

The part that is so understated you almost feel like you should bring it up is that your coin is not only interacting with your hand; it's interacting with the machines that are designed around its geometry as well. A 2021 paper titled “Design of a Low Cost Coin Acceptor for Vending Machine,” by Ho, Zulkipli and Shaari from Tun Hussein Onn University of Malaysia, published in the journal Progress in Engineering Application and Technology, found that coin-validation systems are based on the accurate measurement of a coin’s physical characteristics such as diameter, thickness, and the profile of the coin's edge, to differentiate between coins and junk.

When a coin's edge structure deforms, twists, or wears unevenly, it becomes a coin that gets stuck in a laundromat machine or is simply rejected by a parking meter. By keeping that edge profile consistent over a coin's working life, the rim is doing quiet, ongoing work today: it's a big part of why the vending machines, laundry machines, and parking meters currently in use can still read decades-old coins reliably, without needing to know or care how old any individual coin is.

A design choice we've stopped noticing

It's funny how we don't notice the object enough that we primarily use in our day-to-day life and interactions. However, the raised rim is a reminder that “boring” physical objects often have more design meant for them than the slick apps that they replaced. None of the researchers in the Mint report claimed that the ridge was purely for looks; it's there to address two rather practical, if not chic, issues: wear and stackability. The next time you're given change by a barista, notice the little raised edge. It's not decoration. It's 230 years of applied physics, simply keeping loose change from getting cluttered.