Topics: A first two-dimensional IRT model with an own latent variable for the positively and for the negatively formulated items. A first alternative to the Birnbaum model, Testing the hypothesis: The correlation between the two latent variables is 1, The general structural model for the latent variables and the matrices involved in this model, An alternative way of testing the hypothesis mentioned above. The regression of Xi2 on Xi1 is perfect. (The variance of the residual is 0).