The ten-year Treasury yield minus the three-month bill yield. An inverted curve, sustained for a quarter, has preceded every US recession since 1968 with a lead of six to eighteen months.

Threshold source: Estrella and Mishkin (1996), Federal Reserve Bank of New York. How thresholds are chosen

The lead is long and variable, and the curve re-steepens before the recession arrives, which is why the state here turns to Post-trigger rather than Normal when it un-inverts.