Long-term light curve of the Crab Nebula (the total Syn+IC emission) for the 60 MeV–600 MeV band. The size of each bin is five days. The flux measurements of all bins are plotted as black circles with statistical uncertainties, while the upper limits of a 95% confidence level (only for the bins with insufficient photon statistics) are plotted as brown triangles. The black horizontal line indicates the intermediate-state average flux, while the red and blue lines respectively indicate the thresholds of the “high” and “low” states we define (see the text for detail). Blue vertical bands indicate continuous (≥15 d) “dip” features which are reported in Table 3.