The ten-year minus three-month spot rate on the ECB's yield curve fitted to AAA-rated euro-area government bonds. Inversion sustained over a quarter has preceded the euro-area recessions of 2008 and 2011.
Threshold source: Estrella and Mishkin (1996), applied to the ECB's AAA-rated spot curve. How thresholds are chosen
Two recessions is a short record. The AAA curve is dominated by Germany; peripheral spreads carry the stress that this series does not see.