The rise of "ratings arbitrage"—i.e. pooling low-rated tranches to make CDOs—helped push sales of CDOs to about $500 billion in 2006,[14] with a global CDO market of over USD $1.5 trillion.[34] CDO was the fastest-growing sector of the structured finance market between 2003 and 2006; the number of CDO tranches issued in 2006 (9,278) was almost twice the number of tranches issued in 2005 (4,706).[35]
CDOs, like mortgage-backed securities, were financed with debt, enhancing their profits but also enhancing losses if the market reversed course.[