In this short chapter, we establish some basic facts about the edge-triangle model. We review Pikhurko and Razborov’s results about the geography of the phase space of possible values of pairs of the edge and triangle densities, the so-called Razborov triangle. We derive the unique entropy-maximizing graphon at each point on the boundary, and prove that there is a phase transition across the so-called Erdős–Rényi curve in the middle of the phase space. We compute the optimal graphon along a certain line segment that is disjoint from the Erdős–Rényi curve, answering an old question of Chatterjee and Diaconis. Along the way, we develop several proof techniques that will be repeated throughout the book, in particular the Method of Saturated Inequalities and the Method of Competing Inequalities.