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<p>[QUOTE=&quot;physics-fan3.14, post: 4366624, member: 19165&quot;]Tradition, mostly. When Sheldon made the scale, he did not use every grade. There were jumps. He didn&#039;t use all the numbers in the UNC scale either, he only used 60, 65, and 70. Over time, more numbers got filled in, some numbers were used previously but not anymore (such as F-18). </p><p><br /></p><p>For an extremely well presented account of the history of the grading scale, read this article by Tom Delorey. He was intimately involved in developing the current scale: <a href="https://coinweek.com/education/coin-grading/the-early-days-of-numerical-coin-grading/" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://coinweek.com/education/coin-grading/the-early-days-of-numerical-coin-grading/" rel="nofollow">https://coinweek.com/education/coin-grading/the-early-days-of-numerical-coin-grading/</a>[/QUOTE]</p><p><br /></p>

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