There is a qualitative gap between mathematical bifurcation diagrams and experimentally observed force versus displacement curves. This chapter introduces a procedure to fill this gap by extending the Koiter law for describing imperfection sensitivity at a pitchfork bifurcation point. This procedure is applied to a truss dome structure and sand specimens. Chapters 2 and 3 are foundations of this chapter. This chapter is extended to a system with group symmetry in Chap. 9 and is applied to the imperfect bifurcation of cylindrical specimens in Chap. 14.