Fiat-Backed Stablecoins: USDT, USDC and the Reserve Question

Last lesson you met the four families of stablecoins and the magic trick that holds a peg: arbitrage. Now we zoom all the way in on the family that holds the most money by a country mile — fiat-backed stablecoins, the USDT (Tether) and USDC (Circle) you’ll see quoted in every trading pair.

The design is so simple it sounds like a trick question. For every coin in circulation, the issuer holds one real dollar — or one dollar’s worth of safe, dollar-denominated assets — somewhere off-chain. The on-chain coin is just a claim ticket. Think of a coat-check at a fancy restaurant: you hand over your coat (a dollar), you get a little numbered ticket (a coin) you can pass around all night, and whoever shows up with the ticket gets the coat back. The ticket is only worth a coat because there’s a real coat in the back room. Everything interesting about fiat-backed stablecoins is a question about that back room.

How fiat-backed coins work — mint, redeem, repeat

Behind every fiat-backed stablecoin sits a company — the issuer. Tether Limited issues USDT; Circle issues USDC. They are the coat-check clerk, and they run two operations that define the entire system.

• Mint. You (well, an approved large client) wire the issuer 1 real dollar. The issuer creates 1 brand-new coin on-chain and sends it to you. Coins outstanding go up by one; reserves go up by one dollar.
• Redeem. You send 1 coin back to the issuer. It wires you 1 dollar and burns the coin — destroys it permanently. Coins outstanding go down by one; reserves go down by one dollar.

That’s it. One dollar in, one coin out; one coin in, one dollar out. In principle the coin supply and the reserve pile move in perfect lockstep, so the system stays fully reserved: every coin is always matched by a dollar of backing.

This direct in-and-out is the engine behind the arbitrage you met last lesson. Recall the peg-defending loop: if a coin slips to $0.99 on an exchange, an authorized party buys 1,000,000 coins on the open market for $990,000, walks them straight to the issuer, and redeems them for $1,000,000 in dollars — a tidy $10,000 risk-free profit. Their buying pressure shoves the price back toward $1. The arbitrage only works because redemption at $1 is real and reliable. Kill the redemption and the peg loses its anchor.

What actually backs them — it’s not a pile of cash

Here’s where the coat-check analogy gets uncomfortable. The clerk did not leave your actual coat hanging on a hook. The clerk took your coat to a tailor, who is using it as collateral to… okay, the analogy breaks, but the point stands: “backed by dollars” rarely means dollars sitting still.

Holding idle cash earns nothing, and these issuers hold tens of billions. So instead they hold reserves — a portfolio chosen to be safe and to throw off interest. A healthy fiat-backed reserve is dominated by:

• Short-dated US Treasury bills (“T-bills”) — IOUs from the US government that mature in days or weeks. As close to risk-free and as liquid as a financial asset gets: you can sell them for cash almost instantly, at almost exactly face value, even in a crisis.
• Cash and bank deposits — actual dollars at banks, for instant redemptions.
• Overnight repurchase agreements (“repos”) — ultra-short secured loans, cash-like.

The danger zone is everything else a less careful issuer might reach for to juice returns: commercial paper (short-term corporate IOUs), corporate bonds, secured loans, even other crypto. On a calm day these all “add up to a dollar.” The problem is that quality only matters on a bad day.

Why does reserve quality matter so much? Because a stablecoin’s promise isn’t “I’m worth a dollar on a quiet Tuesday.” It’s “I’m worth a dollar the moment you want out — including the moment everyone wants out at once.” When redemptions surge, the issuer must sell reserves for cash fast. T-bills sell instantly at full value. Commercial paper, corporate bonds, and crypto can be illiquid (no buyer right now) or sell at a steep loss exactly during a panic. So a coin that is “100% backed” on paper can still fail to redeem at $1 under stress — the assets are there, but they can’t be turned into a dollar in time, or only at $0.90 on the dollar. Full backing on paper is necessary but not sufficient; liquidity and credit quality are what make the peg hold when it’s tested.

Attestations vs audits — don’t confuse the two

So reserves are everything. How do you, sitting at home, actually know the back room is full? You can’t see it yourself, so you rely on a third party to vouch — and there are two very different levels of vouching, routinely conflated.

• An attestation is a snapshot. An accounting firm looks at the books on one specific date and signs a statement like “on March 31, reserves were greater than or equal to liabilities.” It’s real, but it’s narrow: it says nothing about the day before or after, doesn’t deeply probe whether the assets truly exist or who else has a claim on them, and is limited to what management showed.
• A full audit is deeper and rarer. An auditor examines controls, traces assets, considers what could go wrong over a period, and issues a formal opinion. It’s a much higher bar — and far less common in stablecoin land than most people assume.

The track records differ. Tether historically published attestations rather than full audits, and was fined by US regulators (the CFTC and the New York Attorney General) for past periods in which its “fully backed” claims were not accurate — a useful reminder that “attested” is not a guarantee. Circle publishes monthly reserve reports for USDC and has leaned toward more frequent, more detailed disclosure.

The centralization tax — one company, one switch

A fiat-backed coin lives on a public blockchain, so it feels like the permissionless crypto you came for. But it is permissioned at the edges: a single company controls minting, redemption, and — crucially — a freeze switch. This is the price you pay for that beautifully simple, tightly-pegged design. Call it the centralization tax.

What the issuer can do that a truly decentralized system cannot:

• Freeze / blacklist addresses. Both USDC and USDT have frozen funds at addresses, typically at law-enforcement or court request (sanctions, hacks, fraud). Your coins can become unmovable bricks without any action on your part. The issuer flips one switch and your “bearer asset” stops bearing.
• Pause redemptions. If the bank holding the reserves wobbles, or a regulator orders it, the redemption window can slam shut — and a stablecoin you can’t redeem is a stablecoin that can de-peg.
• Be a single point of failure. One issuing company, often a handful of banking partners, and a home regulator. When Silicon Valley Bank failed in March 2023, Circle had a few billion dollars of USDC reserves stuck there, and USDC briefly de-pegged to around $0.87 over a weekend until the deposits were guaranteed. One bank’s bad weekend, the whole coin’s peg.

You are trading decentralization for simplicity and a tight peg. Whether that’s a good trade depends entirely on what you’re using the coin for. Here’s the deal laid bare, fiat-backed vs an idealized decentralized coin:

When fiat-backed makes sense

For all that centralization grumbling, fiat-backed coins dominate for excellent reasons. They have the deepest liquidity (USDT and USDC are the most-traded assets in all of crypto), the tightest peg (because direct $1 redemption makes arbitrage almost frictionless), and the simplest mental model (a dollar per coin in the back room — no clever financial engineering to blow up).

That makes them the default for trading and payments: the unit you price pairs in, settle with, and move across exchanges. If you want a dollar that behaves like a dollar and you’re willing to trust a regulated company to hold the reserves, this is the family for the job.

The bill for all that comes due as counterparty trust. You are trusting that the reserves are real, high-quality, and redeemable; that the company won’t freeze you on a bad day; and that its banks and regulators stay friendly. Decentralized coins exist precisely because some people aren’t willing to sign that contract — which is exactly where we go next.

Key Takeaways

Next up: crypto-backed stablecoins — DAI and the world of over-collateralized dollars, where the back room is a public smart contract instead of a company’s bank account, and you lock up more than a dollar of crypto to mint each dollar of coin.