Credit Suisse's rescue had a sting in its tail for the banking crisis. Here's what you need to know about AT1 bonds.

• Credit Suisse was rescued by its longtime rival UBS on Sunday.
• As part of the deal, the Swiss regulator wrote down the value of the bank's so-called AT1 bonds to zero.
• Here's everything you need to know about AT1s and why they could fuel further turmoil in financial markets.

UBS bailed out its longtime rival Credit Suisse in a historic $3 billion deal Sunday – but there was a sting in the tail for holders of the beleaguered Swiss bank's so-called Additional Tier 1 bonds.