Posted By:
petecldI promise, no tangents this time.
In a nut shell bat ears are caused by difference in hand pressure applied to the straight edge when cutting.
A cutting machine applies even pressure along the entire length of the cut. Humans aren't that consistent. When trimming a sliver like John said the tendency is to apply heavy pressure when you first start to cut - because mentally you're being extra careful - and then lighten your hand pressure as you cut - you're gaining confidence in your cut and mentally preparing to stop cutting. This ever so slight lightening of pressure allows the blade to move slightly and when that happens you get bat ears.
Also know that NO mechanical cutter for the printing industry will cut a curve or arc. A sheet may shift when cutting and give you a arched cut but the sheet will also show damage. In my opinion any card with an edge that has an edge with a curve of any length should be considered trimmed. Considering the volume in which Baseball cards are printed and were printed, and I'm including 19th century issues, there is no reason for a card to have anything but a straight edge. And as John mentioned the edges should be parallel.