The 48 Hour Rule refers to the part of the mortgage allocation process related to the purchase and sale of Mortgage Backed Securities (MBS) to be announced (TBA).
The 48 hour rule stipulates that the MBS seller notifies the buyer of the details of the underlying mortgages that make up the MBS by 3:00 pm. Eastern time, 48 hours before the settlement date.
The Securities Industry and Financial Markets Association (SIFMA) applies the 48 hour rule.
When MBS are traded on the secondary market, the underlying mortgages are unknown, which helps facilitate trading and liquidity.
Certain information is negotiated when making a transaction with MBS, such as price, denomination and coupon, but not the underlying mortgages.
The TBA market is the second most traded secondary market after the US Treasury market.