For the past year I've been involved in a little research project to determine the thickness of early playing cards. I've contacted several institutions and received varying degrees of assistance. The hands down winner is the Beinecke Library at Yale University. I asked for some measurements and got back a study. I cannot begin to say enough good things about the staff there. I will remind everyone that the images are the property of the Beinecke Library and if you re-use any of the information within this study you must credit the Library appropriately.