Here you can see how the marks on the back of the cards really look like.
The video on the side (the thumbnail with the play icon on it on the left) shows how you will see the marks on the back of the cards while you wear our lenses or glasses.
Different brands of card manufacturers are shown, as well as different back sides of the cards in different colors. The marks of the cards also vary - some are marked with a big letter or number in the middle of the card or 2 in the corners. Others use special marks on the border frame of the cards, that follow specific patterns and are easy to learn.