AT1, or additional tier 1 bonds, are a type of perpetual bonds that banks use to augment their core equity base. They do not have a maturity date. Banks have a call option that permits them to redeem these bonds after a certain period. They are subordinate to all other debt and senior only to equity. If the capital ratios of a bank falls below a certain percentage or in case of an institutional failure, the rules allow the issuer to stop paying interest or even write down these bonds. If a bank reaches the point of non-viability, AT1 bonds are typically the first part of debt that is written down.