A lot of people will call this sort of error a strikethrough, but I'm a bit pickier in my choice of descriptions. When I think of a strikethrough, I associate it with a one-time, immediate occurrence. For example, a coin that is created by the strikethrough of a piece of wire is usually a one-of-a-kind error. The same is usually true for a dropped element. But, grease and debris doesn't fill the incuse part of a die all at once. It is a gradual accumulation that is compacted by continually striking planchet after planchet after planchet. That is why I refer to it as "the result of a grease-filled die". This doesn't mean that I don't understand what other people are saying when they describe a coin as "struck through grease". It's just the Virgo in me. Chris