The Bogleheads' Guide to Investing

It’s important to understand that bonds and bond funds have a low correlation (they don’t always move in the same direction at the same time) to stocks, so bonds can be a stabilizing force for a portion of your portfolio. For example, in the bear market of 2008, while equity fund losses of 30 percent to 60 percent were common, Vanguard’s Total Bond Market Index Fund gained 5.05 percent.