While we're on the subject, there's two other relationships to monitor.
One is the ratio of investment grade corporate bonds, rated AAA, with US Treasury bonds. That ratio has been going down, in a "flight to safety."
The other is the relationship between investment grade corporate bonds and high yield corporate bonds. That ratio has been going up, also in a "flight to safety." (click for updates).
Remember: these are not just squiggles on a chart. They have profound implications for the health of businesses and for future levels of unemployment.