BOSTON ( TheStreet) -- Bond investors use Treasury yields as a benchmark because these government-issued notes are as risk-free as a security can be.The performance of a corporate bond is often measured against a comparable Treasury. The difference, or yield spread, can seem like an esoteric gauge to many investors. But understanding spreads can help investors identify bonds headed for a correction. The spread for a bond is simply the difference between its yield and the yield of a comparable Treasury security. The spread between the two securities reflects the risks they pose, such as those related to credit, liquidity and market forces. Spreads should be consistent for bonds with similar credit ratings, one of the biggest factors in determining the value of a bond. Fixed-income investors use spread changes to help them pick attractively priced bonds. Spreads typically return to their long-term averages over time. So when a spread expands above the average, corporate securities look cheap because the rates on these bonds have increased, pushing down the prices. The chart below shows the 10-year corporate bond spread for AAA-rated and BBB-rated securities over 10-year Treasury yields, and the historical average of the spread. During the turbulence of 2008 and 2009, these spreads reached unprecedented levels.