The mortgage backed securities r packages of loans, with varying rates of interest. For example, there r ''A'' packages, with full documentation to ''C'' packages with sub-prime, alt-A, etc., more risky but with a greater reward. The mortgage market is in the trillions, & the buying & selling of these mortgages is what keeps cash flowing through the system. If one part of that gets clogged up, the market becomes tighter, that is why we have a liquidity crisis now. Most people r paying their mortgage, in some parts of the country it is 99.9%, in other, overheated areas, it is 90%. This disparity is what prevents the smooth buying & selling of these mortgages. The problem is that the percentage of defaulted mortgage obligations, while small, r coming into a system that is not designed to handle even that number of defaults, & it is causing banks & securities dealers to experience a liquidity crisis. If no one wants to buy mortgages, u do not have fresh capital to make more.