Virtually spot or mark free, a nearly flawless beauty whose defects are barely perceptible to the naked eye. If defects exist in focal areas like the cheek they must be minimal and compensated for by the rest of the coin's superbness. Must have an above average strike and full original mint lustre. The fields at this grade usually have a frostier, more pristine look than MS65s.
MS66 was something graders had to get a feel for, a new level between gem and superb gem, and it wasn't often used in the early days of certification. As a result many MS65s have been cherrypicked for their outstanding quality and are now MS66.
The best can be impossible to upgrade given the low populations at this level and higher, particularly the more difficult dates. The worst have some suberb qualities but too many distracting problems to make the jump from MS65.