If you search for information on this index, you will see that it is used a great deal on the web but I have not found a simple explanation for what it actually is and the precise role that if plays in finance. Here's what I have been able to piece together.
The CMBX is an index published by a company called Markit . It is simply an index that tracks performance of several baskets of bonds. The CMBX AAA index tracks the yield on a group of triple A bonds that are backed by real estate loans.
So, first we need a quick explanation of yield. Current Yield is the ratio of the annual interest payment and the bond's current price. If a bond costs $100 dollars and has an annual coupon Interest payment) of $10 the yield is 10%. If the price falls to $80, then the yield increases to 13%. The value of the bond would fall if the market believed that the bond became more risky. Riskier investments have a higher yield to compensate the holder for the risk of default.
To make things more confusing, the CMBX does not track the bond yields directly. The index is expressed as a 'spread'. In this case this is the difference (spread) between the yield of the bonds being tracked and the current yield offered by government bonds (commonly called the 'risk-free' rate).
The result is that when the bond prices fall, the spread increases and the index goes up. A high index value means that the financial community sees these bonds as being at a higher risk of defaulting. We can see from this chart that the spread for triple A rated bonds has increased from less than 0.5% in October to over 2% in February.
And how is the CMBX used? It is not a product you can buy directly. See CORRECTION in comments. This index is used as a benchmark for pricing other financial products: Credit Default Swaps. A CDS acts like an insurance policy on a bond. The seller of the product will assume the default risk of a bond. If the bond defaults, the seller will pay the buyer the value of the bond. For this insurance, the buyer pays the seller a periodic premium that is related to the riskiness of the bond. And how do they agree on the riskiness of the bond? The use the CBMX index!
You don't have to own the bond in question to buy a CDS against it. If you want to, you can bet against the commercial real estate industry. When real estate companies default, you will collect a windfall!



