Broke The Buck

When a money market mutual fund's net asset value (NAV) drops below $1 per share. Money market funds aren't federally insured like bank deposits; therefore, fund assets have an implied promise to preserve capital at all costs and preserve the $1 floor on share prices. These funds are regulated by the Securities and Exchange Commission and Rule 2a-7 restricts what they can invest in based on credit quality and maturities with the hope of ensuring principal stability.

Breaking the buck is an extremely rare event that money market fund managers always want to avoid, but it can occur if the underlying fund investments (which are generally assumed to be completely safe) significantly drop in value. This can happen if the underlying investments suffer large losses, such as defaults or strong moves in interest rates. Several funds reached or approached this critical point (from an investor faith standpoint) during the credit crisis that occurred as a result of a drop in mortgage-related assets beginning in 2007.