The notion of “Joker" is appeared in the calculation of Ext groups of certain modules over the Steenrod algebra in a paper “Uniqueness of $BSO$” of Adams-Priddy. Recently, A. Baker constructed a space whose mod 2 cohomology is isomorphic to Joker. The aim of this talk is to provide some background to understand a significance of Joker by introducing the notion of stable category of modules and the Picard groups of invertible stable modules following works of Adams and Margolis.