Okay, so in this short video we're going to be looking at the phase and lever rules. The phase rule is really important because it defines the conditions of equilibrium. It's one of those rules that when I first saw it I thought people were just making up, but actually these equations are derived directly from the Gibbs rules of thermodynamics. So the phase rule can be written P plus F equals C plus 2. So P is the number of phases. F is the number of degrees of freedom and C is the number of components. Now for solid state systems we can generally ignore any significant vapor phase, so we can simplify this down so that P plus F equals C plus 1. This is what we call the condensed phase rule. So this simplifies further for us because in our systems we're only looking at binary systems and so C is always going to be 2 because we've got two components. So therefore this simplifies even further for us P plus F equals 3. So the idea of this is that we can look at any point on our phase diagram and determine whether it makes sense because we can know what the conditions of equilibrium should be at every point. So for example if we have the liquid phase up here, our all points in this phase field we just have a single phase liquid. So P equals 1. That means P plus F equals 3, that means the number of degrees of freedom must be 2 in the liquid field. Okay, that means that for every temperature and composition we have a different state for the liquid. Also in these two fields we know we have two phases. These are the primary phase fields of A and the primary phase field of B. So finally we'll go this up here. We know we've got 8 plus liquid in here and B plus liquid in here. So P equals 2 and F equals 1. So we have 1 degree of freedom. In here we have 2 phases again so P equals 2 F equals 1. Anywhere along the eutectic line we have 3 phases so P equals 3 which means we have no degrees of freedom. So that means we have to fix all of the variables to define the system at any given point.