Because MS-66 is an odd grade.
The difference between an AG-3 and G-4 coin are clear. The difference between a MS-65 and MS-66 coin are not too clear and are very subjective. One man's MS-65 is another man's MS-66. Also, especially for early coins, MS-66 coins are almost unknown, or fetch extreme prices.
For most coin books, there's no need to include the entire tier from MS-60 to MS-70 because the grades there are so subjective. The difference between a MS-63 and MS-65 may be quite easy to see, same with a MS-65 and MS-67 but the difference between an MS-65 and MS-66 are very slight.
The absence of an MS-66 grade in coin books is due to grading standards established by third-party grading companies like NGC and PCGS. These companies use a scale that ranges from MS-60 to MS-70, with MS-70 representing a perfect coin and MS-60 showing noticeable wear. The decision to omit an MS-66 grade from coin books may have been made to simplify the grading system and reduce ambiguity.
MS66 is a specific grade for a coin's condition on the Sheldon Scale, which grades coins on a scale of 1-70. MS stands for mint state, and a 66 out of 70 is nearly flawless.
To set things straight and so that you can find your coin when doing searches, that's ST GAUDENS and NO MOTTO. Numismedia lists a retail fair market value of an MS66 no-motto St. Gaudens double eagle at roughly $3800 for a Philadelphia coin, and 10 times that for a Denver coin. In any case you need to be sure that your coin has been certified by one of the major grading services. MS66 is a stratospheric level for a coin and only a trained expert can assign that grade reliably.
MS66 is a grade of a coin, rather than a mark on a coin."MS" stands for Mint State, meaning the coin looks like it did when it was first minted. "66" is a numerical gradation within the broad category of Mint State. For MS coins, the numerical grade can be anywhere from 60, the minimum acceptable condition for Mint State, to 70, meaning an absolutely perfect coin. Very few coins rate even 68 let alone 70, so an MS66 nickel would appear to be brand-new and in addition would be quite sharp and well-struck. As of 08/2015 the retail value for a 1908 nickel in that condition would be around $2600.
You have to know what it is. What country, what date, what denomination. Then you have to grade the condition. There are books that explain how to do this.
15 gold coins
August 10, 2009 The 1923-D Saint-Gaudens Double Eagle in MS66 grade has a PCGS value of $4,100.
The grade of a circulated coin is based oc how much wear it has.
A Proof coin is a particular issue of a coin not a grade of condition.
8-3-11>>> The 1888-O Morgan dollar is a common New Orleans Mint coin. Most uncirculated coins don't grade higher than MS-63. A coin in MS-66 is a "conditional rarity" and has a value of more than $50,000.00. The coin would have to be certified by one of the top 3 grading services to have this value.
PF-69 is not a value, it's a grade. But it does affect the value, the higher the grade the more value a coin has. A PF-69 coin is one grade below a "Perfect" proof coin.
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The year of any coin does not determine the grade that's judged by how much wear the coin has.