Corporate bonds have credit ratings that indicate their default risk. Themost highly rated is AAA (almost no default risk), followed by AA, A, BBB, BB, B, and C. Bonds with ratings at or better than BBB is called investment grade; worse bonds are called high yield or junk bonds. Many institutional investors (e.g. pension funds) are legally prohibited to hold high-yieldfunds. Note that credit ratings do go up or down over time. If a bond gets downgraded from BBB to BB, all those funds will have to sell.

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