The scales run AAA, AA, A, BBB, BB, B, CCC, CC, C, D at Standard & Poor's and Fitch, and Aaa, Aa, A, Baa, Ba, B, Caa, Ca, C at Moody's. Each letter band except the extremes has three notches, so BBB plus, BBB and BBB minus are one notch apart.
Ratings are slow and backward-looking relative to market pricing, and spreads usually move well before a downgrade lands. They still matter because so many mandates, capital rules and index definitions are written in terms of them.
Example: an issuer is downgraded from BBB minus to BB plus, a single notch, but it crosses the investment-grade boundary. Index-tracking holders must sell, and the bond drops four points on a change that was already priced in spread terms.
Related: investment-grade, high-yield, rating-agency, fallen-angel, default