I was perusing the online page of NGC grading criteria and noticed the word "hairlines" used in describing some UNC grades:
MS/PF 66 Very well struck with minimal marks and hairlines.
MS/PF65 Well struck with moderate marks or hairlines.
MS/PF64 Average or better strike with several obvious marks or hairlines and other miniscule imperfections.
Can someone post an image of such hairlines? My only encounter with the term is with "details" coins.
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Edit: shoulda looked before leaping, I found a great discussion here on CCF:
http://goccf.com/t/7261