Additional Tier 1 capital under Basel III is a layer of bank funding that absorbs losses while the bank is still operating. It sits between common equity and Tier 2 debt in the capital stack, filling the 1.5% gap between the 4.5% Common Equity Tier 1 (CET1) minimum and the 6% total Tier 1 requirement that all banks must meet. If the issuing bank’s capital deteriorates past a contractual threshold, AT1 instruments automatically convert into shares or get written down, forcing private investors to bear the loss instead of depositors or taxpayers.
Where AT1 Sits in the Capital Stack
Basel III sets three minimum capital ratios, each measured against a bank’s risk-weighted assets:1Bank for International Settlements. FSI Summaries – Definition of Capital in Basel III
- CET1 of at least 4.5%, made up of common shares and retained earnings
- Total Tier 1 (CET1 plus AT1) of at least 6%
- Total capital (Tier 1 plus Tier 2) of at least 8%
AT1 exists to fill the gap between the CET1 floor and the Tier 1 requirement, which means banks need AT1 instruments equal to at least 1.5% of risk-weighted assets. On top of these minimums, a capital conservation buffer of 2.5% in CET1 applies, and dipping into it restricts what the bank can pay out.2Bank for International Settlements. RBC30 – Buffers Above the Regulatory Minimum
The logic is sequential. Losses eat through CET1 first, then AT1, then Tier 2 debt. Each layer creates distance between the bank’s troubles and the point where depositors or public funds would be exposed. AT1 is called “going-concern” capital because it absorbs losses while the bank remains open, unlike Tier 2, which only comes into play in liquidation.
What Instruments Qualify as AT1
Banks primarily use Contingent Convertible bonds, known as CoCos, to meet their AT1 requirements. CoCo issuance took off after 2012 as banks came under pressure to boost Tier 1 capital, and the volume has grown substantially since.3Bank for International Settlements. CoCos: A Primer Investors collect regular coupon payments but accept a defining risk: if the bank’s capital falls below a trigger level, their bonds either convert into common shares or lose value through a write-down.
Perpetual subordinated notes are the other main AT1 instrument. They have no maturity date and rank below senior creditors in a bankruptcy. Both types share the same core feature. They look and pay like debt during normal times, and transform into loss-absorbing equity when the bank comes under stress. Institutional buyers accept the deal because AT1 pays significantly higher yields than senior bank debt.
What Makes an Instrument Count as AT1
The Basel Framework sets detailed criteria an instrument must meet before it can be classified as AT1. The rules exist to ensure the instrument genuinely absorbs losses rather than functioning as ordinary debt in a different label.
Perpetual With No Incentive to Redeem
An AT1 instrument must have no maturity date and no step-ups or other features that would push the bank toward buying it back. Banks can include a call option, but only after a minimum of five years from issuance, and even then the bank needs prior supervisory approval. It must also either replace the called instrument with capital of equal or better quality, or show that its capital position remains well above the minimums after the call.4Bank for International Settlements. CAP10 – Definition of Eligible Capital Banks are prohibited from doing anything that creates a market expectation the call will be exercised.
Fully Discretionary, Non-Cumulative Coupons
The bank must have full discretion at all times to cancel coupon payments, and cancellation cannot count as a default.4Bank for International Settlements. CAP10 – Definition of Eligible Capital Missed coupons are gone permanently. The bank has no obligation to make them up later, and skipped payments cannot trigger restrictions on the bank beyond limiting dividends to common shareholders.5Bank for International Settlements. Basel III Definition of Capital – Frequently Asked Questions Any arrangement to compensate investors for unpaid coupons is explicitly prohibited.
Subordination to All Senior Claims
AT1 instruments must sit below depositors, general creditors, and the bank’s subordinated debt.4Bank for International Settlements. CAP10 – Definition of Eligible Capital They cannot be secured or backed by any guarantee that would effectively move them up the repayment ladder. In a wind-down, AT1 holders collect only after every senior claim has been paid in full.
How Loss Absorption Actually Works
Two types of trigger clauses can activate loss absorption on an AT1 instrument, and they operate on different logic.
The Quantitative Trigger
Every AT1 instrument classified as a liability must include a contractual clause that activates a write-down or conversion if the bank’s CET1 ratio falls below a specified level. The Basel Framework sets the floor at 5.125% CET1, though banks can set higher contractual thresholds. A significant portion of outstanding AT1 bonds trigger at 7% CET1.6Bank for International Settlements. FSI Briefs No 21 – Upside Down: When AT1 Instruments Absorb Losses Before Equity When the trigger is breached, the write-down or conversion happens automatically, without any court order or regulatory decision. The amount absorbed must be at least enough to restore the CET1 ratio back to the trigger level.
The Point of Non-Viability Trigger
The second trigger is discretionary and sits with the regulator. The Basel Framework defines the point of non-viability (PONV) as whichever comes first: the regulator deciding a write-down is needed to restore the bank’s viability, or a decision by the public sector to inject support.6Bank for International Settlements. FSI Briefs No 21 – Upside Down: When AT1 Instruments Absorb Losses Before Equity A PONV trigger can fire even when the bank’s reported CET1 ratio is still above the quantitative threshold, if the regulator concludes the bank’s position no longer supports market confidence. This is the more unpredictable of the two because activation depends on regulatory judgment.
Conversion Versus Write-Down
When either trigger fires, the bond terms dictate the outcome. One option is mandatory conversion into common shares, which instantly boosts the bank’s equity by transforming debt into ownership stakes. The other is a principal write-down, which reduces or eliminates the face value of the bond. Write-downs can be permanent, wiping out the investor’s principal entirely, or temporary, allowing value to be restored if the bank recovers. Most AT1 contracts specify one mechanism, not both.6Bank for International Settlements. FSI Briefs No 21 – Upside Down: When AT1 Instruments Absorb Losses Before Equity
The hierarchy is not always as clean in practice as it looks on paper. If a bank’s CET1 ratio drops below 5.125% but the bank has not been placed in resolution, AT1 bonds may be written down while common shareholders still retain some value. Regulators have flagged this “upside down” scenario as an unintended consequence of how trigger levels interact with resolution timing.6Bank for International Settlements. FSI Briefs No 21 – Upside Down: When AT1 Instruments Absorb Losses Before Equity
Coupons Can Stop Before Any Trigger Fires
Well before an AT1 trigger event occurs, coupon payments can be restricted by the capital conservation buffer framework. When a bank’s CET1 ratio falls into the buffer zone between 4.5% and 7.0%, it faces escalating constraints on how much of its earnings it can distribute. The lower the CET1 ratio drops within that band, the smaller the share of earnings the bank is allowed to pay out. Near the bottom of the range, the bank must conserve all of its earnings and cannot make any distributions at all.2Bank for International Settlements. RBC30 – Buffers Above the Regulatory Minimum
AT1 coupon payments count as capital distributions under these rules.7Federal Register. Regulatory Capital Rule: Category I and II Banking Organizations So an AT1 investor can lose coupon income well before the write-down or conversion trigger fires. The bank does not need to invoke discretionary cancellation. The buffer constraints impose the restriction automatically, and the effective risk of missed coupons begins at a CET1 level well above the 5.125% trigger floor.
The Credit Suisse Precedent
The largest real-world test of AT1 loss absorption happened in March 2023, when Swiss regulator FINMA ordered the complete write-down of Credit Suisse’s AT1 bonds as part of the bank’s emergency acquisition by UBS. FINMA relied on two bases. The contractual terms of the bonds provided for a total write-down upon a “Viability Event” such as extraordinary government support, and an emergency ordinance enacted by the Swiss Federal Council authorized FINMA to order the write-down.8FINMA. FINMA Provides Information About the Basis for Writing Down AT1 Capital Instruments
The write-down wiped out approximately 16.5 billion Swiss francs in AT1 bonds while Credit Suisse shareholders received shares in UBS. That inversion of the expected loss hierarchy sparked outrage among AT1 bondholders, who argued they should not have been wiped out before equity holders. A Swiss court later ruled the write-down was unlawful, though the practical implications of that ruling for bondholders remain unresolved. The episode demonstrated that regulatory powers can override the contractual hierarchy investors expect, and it reshaped how the market prices AT1 risk. Since then, investors pay far closer attention to the legal framework of the issuing jurisdiction and the exact contractual terms governing PONV triggers.
Why Banks Use AT1 Rather Than Just Issuing More Equity
If AT1 capital sits near the bottom of the loss hierarchy and regulators can force it to absorb losses, it might seem simpler to issue more common shares. The reason banks don’t comes down to cost. New share issuance dilutes existing shareholders, depresses the stock price, and faces board resistance. AT1 instruments avoid this dilution during normal operations because they function as debt with regular coupon payments. In several jurisdictions, regulators treat AT1 coupons as interest payments for tax purposes, letting banks deduct them from taxable income. That makes AT1 meaningfully cheaper to service than common equity, where dividends come from after-tax profits. The tax treatment varies by jurisdiction. Where regulators classify AT1 as equity, coupons are treated like dividends and provide no tax advantage.
For the largest global banks, AT1 instruments also count toward Total Loss-Absorbing Capacity (TLAC) requirements, which apply to global systemically important banks and are measured against both risk-weighted assets and total leverage exposure.9eCFR. 12 CFR Part 252 Subpart G – External Long-term Debt Requirement, External Total Loss-Absorbing Capacity Requirement and Buffer AT1 issuance helps these banks meet the elevated requirements without relying entirely on common equity or eligible long-term debt.