This chart is making the rounds of the professional commentariat.
I don’t generally comment on stuff like this, but I think everyone is missing the point (especially Felix Salmon).
The chart shows that AAA ratings accounted for an ever-larger portion of total ratings leading into the crisis. One other thing was going on during that time, namely that credit standards were rapidly deteriorating.
Thus, the percentage of issuances rated AAA were increasing as the assets backing the issuances were of decreasing quality. IMHO, if you can explain how this seemed reasonable to the best and brightest, you can explain the crisis.
Understanding all this is important now, because the official narrative of the financial crisis is still being written.