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Joe_G.SGC has explained the following very general rules with me. In each case below, the grades represent the highest possible on an otherwise clean, even NrMT card.
Any mark by a pen or pencil, even if only 1/4" long will grade no better than SGC50
One line of writing (such as a name) will grade no better than SGC40
Two lines of writing no better than SGC30
Three lines no better than SGC20
Four + lines and you're down to SGC10
I recently had a handful of otherwise beautiful OJs grade SGC20 - SGC30 because of the writing on the back. With PSA, some of my SGC20 - 50 cards would grade much higher with an MK notation (mark). I wish SGC would offer similar grading on blank backed cards such as OJs.
I will continue to use SGC however, even if it means lower average grades.