Fifteen years ago, Lehman Brothers collapsed. Financial giant Lehman Brothers declared bankruptcy in the fall of 2008 with $613 billion in debt, putting thousands of employees out of work and sending an already recessionary economy into a tailspin.

Lehman's dramatic fall was largely due to millions of risky mortgages propping up an unstable financial system. Homebuyers with mortgage payments they could not afford to be defaulted on their loans. When it happened that sent shockwaves through Wall Street and left these borrowers vulnerable to foreclosures.

Borrowers often assumed before the financial crisis that if a lender made a loan, they would be safe, but that proved to be wrong. Still, there are concerns that some of the financial measures put in place after the 2008 financial crisis to protect the mortgage market will not be around forever.

Founded in 1844 as a department store, Lehman Brothers grew into a major investment banking empire. By the early 2000s, Lehman began making mortgage loans and offering mortgage-backed securities, MBS, along with other Wall Street investment banks that also got into what looked as an extremely lucrative business back then.

As we know, the major problem was that many of these mortgages were subprime, with questionable terms and low or no down payments. With little regulation, lenders had to compete with each other to sell the most favorable mortgages with minimal requirements.

Nowadays, nearly the only symbol of the former investment colossus survived, but this symbol is intangible. Lehman Brothers Aggregate Bond Index (The Agg Index), now also known as the Bloomberg Barclays U.S. Aggregate Bond Index (but its original name is still more prevalent), is a benchmark index created back in history by the Lehman Brothers based on their family of indices: Government/Corporate Bond Index, Mortgage-Backed Securities Index, and Asset-Backed Securities Index, including securities that are of investment-grade quality or better, have at least one year to maturity, and have an outstanding par value of at least $100 million. The Agg Index is to the bond market what the Wilshire 5000 Total Stock Index is to the equity market.