What Are Stablecoins? How They Hold a Dollar (and How They Break)

TL;DR

• A stablecoin is a crypto token built to hold a steady value, almost always about one US dollar, so you can use crypto without the price swinging under you.
• There are three main kinds, and they hold their dollar in very different ways: fiat-backed (real dollars in a bank), crypto-backed (other crypto locked up as collateral), and algorithmic (code and incentives, no real backing).
• The algorithmic kind is the one that died: Terra's UST collapsed in May 2022 and wiped itself out in days.
• When a stablecoin slips off its dollar, that is a depeg, and it is the single most important thing to understand before you trust one.
• Every stablecoin that touches the real world needs an oracle: a way for a blockchain, which is blind to the outside world, to be told a real price. That messenger is a favorite target for attackers.

What is a stablecoin, in one sentence?

A stablecoin is a crypto token designed to stay worth about one dollar, so you get the speed and openness of crypto without the wild price swings. Regular crypto like Bitcoin or ETH can move ten percent in a day. That is fine for betting, terrible for paying rent or parking your savings. A stablecoin is the crypto world's answer to that problem: a digital dollar that lives on a blockchain and moves like any other token, but is built to not move in price.

Real examples you will meet constantly: USDC and USDT (also called Tether) are the two biggest. DAI is a well-known one that works very differently under the hood. All three aim at one dollar. How they keep that dollar is where the whole story lives, and where the danger hides.

How do stablecoins stay at one dollar?

Here is the honest answer: a stablecoin does not stay at a dollar by magic, or because someone declares it. It stays there because of a promise plus a mechanism that makes the promise believable.

The promise is: "one of these tokens is worth one dollar." The mechanism is whatever makes traders confident they can always get their dollar back. If people trust that, they treat the token as a dollar, and the price sits at a dollar. If that trust cracks, the price slips, and no amount of branding holds it up.

Think of it like a coat check. You hand over your coat and get a ticket. That ticket is "worth" a coat only because you believe the coat is still back there and you can redeem it any time. A stablecoin is a ticket for a dollar. The entire question is: is the dollar actually back there, and can you really get it? Different stablecoins answer that differently, and that is exactly what separates the three types.

What are the types of stablecoins?

There are three families. The difference between them is simply what is standing behind the dollar.

1. Fiat-backed stablecoins (real dollars in a bank)

This is the simplest kind and the biggest. A company holds real US dollars (and safe dollar-equivalents) in bank accounts, and issues one token for each dollar it holds. USDC and USDT work this way. Hand the company a token, get a dollar back; hand it a dollar, get a token.

• What holds the peg: actual dollars in reserve, plus the ability to redeem.
• The trade-off: you are trusting a company and its bank. This is the least "decentralized" kind, and it only works if the reserves really exist and the company plays honest. That is why reserve transparency and audits matter so much here.

2. Crypto-backed stablecoins (other crypto locked up)

Here there is no company holding real dollars. Instead, users lock up other crypto (like ETH) as collateral inside smart contracts, and the system issues stablecoins against it. DAI is the classic example.

Because crypto collateral itself swings in price, these systems demand more collateral than the stablecoins they issue, an intentional safety cushion. Lock a hundred dollars of ETH, borrow maybe sixty-six dollars of stablecoin. If the ETH drops too far, the system automatically sells the collateral to stay solvent.

• What holds the peg: over-collateralization plus automatic liquidations, all enforced by code, no company required.
• The trade-off: more decentralized, but more complex, and it leans heavily on knowing the true price of the collateral at every moment. Hold that thought. It is where oracles enter.

3. Algorithmic stablecoins (code and incentives, no real backing)

The third kind holds no meaningful reserves at all. No bank dollars, no locked-up crypto cushion. Instead it uses code, a partner token, and trader incentives to try to keep the price at a dollar, minting and burning supply to nudge it back whenever it drifts.

On paper it is elegant: pure math, fully decentralized, no reserves needed. In practice this is the fragile kind, and it is the one that most famously broke. When confidence goes, there is nothing real underneath to catch the fall.

Which kind of stablecoin died?

The algorithmic kind. The clearest, most painful example is Terra's UST in May 2022.

UST was an algorithmic stablecoin paired with a sister token called LUNA. The design let people always swap one UST for a dollar's worth of LUNA, and vice versa, and that swap was supposed to keep UST pinned at a dollar. As long as everyone believed it, it worked, and it grew huge fast, partly because a linked program offered eye-catching returns for holding UST.

Then confidence cracked. UST slipped below its dollar (a depeg), and holders rushed to escape by swapping UST into LUNA. But that escape hatch minted new LUNA to pay people out. The more UST holders fled, the more LUNA was printed, and the more LUNA existed, the less each unit was worth, which meant even more UST needed redeeming, printing even more LUNA. This feedback loop is called a death spiral: the mechanism meant to save the peg was the one destroying it. Within days, both UST and LUNA collapsed toward nothing.

The lesson is not "all stablecoins are dangerous." It is that a stablecoin backed only by confidence and clever code has nothing real to fall back on when confidence disappears. When people talk about the "kind that died," this is it. If you want the mechanism-design view of that failure, what is DeFi sets up the surrounding machinery.

What is a depeg?

A depeg is when a stablecoin stops being worth its target price. A coin meant to hold one dollar starts trading at ninety-eight cents, or ninety, or worse.

Small, brief wobbles happen and often heal, especially for well-collateralized coins, because traders step in to buy the cheap tokens and redeem them for a full dollar, pushing the price back up. That self-correcting pressure is the peg working as intended. A depeg becomes dangerous when the mechanism that is supposed to pull the price back cannot, either because the reserves are not really there, or because the design feeds the panic instead of calming it, as UST did.

So when you evaluate any stablecoin, the real question is not "is it at a dollar right now?" It is "what happens if it slips, and is there something real that pulls it back?" That single question is your filter.

How does a blockchain learn a real-world price? (oracles, in plain English)

Here is a fact that surprises most beginners: a blockchain cannot see the outside world. It has no internet connection, no news feed, no way to know the price of ETH, the price of gold, or even the weather. It only knows what is already written inside it. On purpose. That blindness is part of what keeps it predictable and tamper-resistant.

But crypto-backed stablecoins need outside prices to survive. To know whether someone's locked-up ETH is still enough collateral, the system must know what ETH is worth right now. So how does a blind system get told a real price?

The answer is an oracle. An oracle is a bridge that takes real-world data, like a price, and writes it onto the blockchain so smart contracts can use it. It is the messenger who walks in from outside and tells the blind system, "ETH is worth this much today." The contract then acts on that number: this loan is healthy, that one is under-collateralized, liquidate it.

That is the whole idea. An oracle is not magic; it is just the trusted courier delivering outside facts to a system that cannot fetch them itself. And "trusted courier" is exactly the phrase that should make a security person nervous.

Why an oracle is a juicy target (the auditor's lens)

Here is where my day job as a smart contract auditor changes how you should look at all of this.

Stop and notice what we just built. The stablecoin's most important decisions, is this loan safe, should we liquidate, is the peg holding, all depend on one number delivered from outside. The whole system obeys that number without question. So ask the attacker's question: what if the courier lies?

If an attacker can feed the oracle a fake price, they can make the system act on a lie. Trick it into thinking worthless collateral is valuable, and borrow real stablecoins against garbage. Trick it into thinking healthy collateral is worthless, and trigger unfair liquidations you profit from. The contract is not "hacked" in the movie sense; it is doing exactly what it was told, using a poisoned input. Feed a lie, drain the system.

This is not theoretical. Oracle manipulation is one of the most common ways real DeFi systems get drained, precisely because the oracle is the one soft spot where the outside world meets the code. A blockchain can be flawless and still lose everything if the number flowing into it is wrong. When you look at any stablecoin or lending app now, you should instinctively ask: where does this get its prices, and how hard would it be to lie to it? That instinct is the difference between a user and an auditor. The same soft spots show up when you chase yield, which is why how to earn in DeFi walks through more of these traps.

Are stablecoins safe?

Safer than volatile crypto for holding value, yes, because they are built not to swing. But "safe" depends entirely on which kind and what stands behind it. Run this quick checklist before trusting any stablecoin:

• What backs it? Real dollars, locked crypto, or just code and hope? Fiat-backed and solidly crypto-backed coins have something real underneath. Purely algorithmic ones do not.
• Can you redeem it? A dollar you cannot actually get back is a promise, not a peg.
• Where does it get its prices? If it depends on an oracle, that oracle is a risk, ask how it is protected.
• What happens under stress? The honest test: if a wave of holders tried to exit at once, does the design calm the panic or feed it? UST fed it.

No stablecoin is risk-free. But a coin with real backing, real redemption, and a well-guarded price feed is a very different animal from one held up by confidence alone. Knowing which is which is the skill.

Related questions

Is a stablecoin the same as a normal cryptocurrency? No. Normal crypto like Bitcoin or ETH is meant to move freely in price. A stablecoin is deliberately engineered to not move, pinned to a real-world value like the US dollar so you can use it as steady money on a blockchain.

Is USDC or USDT better than DAI? They solve the same problem differently. USDC and USDT are fiat-backed (real dollars held by a company), which is simple but means trusting that company. DAI is crypto-backed (over-collateralized crypto in smart contracts), more decentralized but more complex. Neither is universally "better," they carry different risks.

Can a stablecoin lose its value? Yes. It is called a depeg. A minor slip often heals when traders arbitrage it back. A severe one, like Terra's UST in 2022, can wipe the coin out entirely if there is nothing real backing the promise.

What is a stablecoin pegged to? Usually the US dollar, one token targeting one dollar. Some target other currencies or assets, but a dollar peg is by far the most common. "Pegged" just means "aimed at and held near" that value.

Why do stablecoins need oracles? Because a blockchain is blind to the outside world. Any stablecoin that depends on real-world prices (especially crypto-backed ones checking collateral value) needs an oracle to deliver that price on-chain. That messenger is also a prime target for attackers, which is why oracle design is a core security concern.

Is fiat the same as a stablecoin? No. Fiat is government-issued money like actual US dollars or euros. A fiat-backed stablecoin is a crypto token that represents and is backed by that fiat, but the token itself lives on a blockchain, not in a bank.

Where to go next

A stablecoin is a simple promise, "one token, one dollar," resting on a mechanism that is anything but simple. The three types hold that promise in different ways, one of them died proving what happens with nothing real underneath, and every one that touches real prices leans on an oracle that an attacker would love to lie to.

The fastest way to make this click is to build the intuition hands-on. Our DeFi foundations course, Your First 90 Days in Web3 by the security firm Zealynx, has a checkpoint that walks through stablecoins and oracles with an auditor's eye, exactly the lens this article sketched. Start the Stablecoins and Oracles lesson below, it is free and needs no account. If you want the wider map first, begin with what is Web3 or crypto assets explained.