Starting from a simple loan (remember them), credit markets have evolved many innovations to cater to an increasing need for leverage (intrinsic firm leverage - levered loans to high-yield bond market; and extrinsic instrument leverage - securitizations and derivatives) for issuers, traders, and investors. However, as the following maze of the history of debt, NY Times [3] shows these have led to many costly crises (and will do in the future...)
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Source: NY Times [3]