Scratching back through my memory - the diagram describes the state of the metal for a given temperature and composition. At X the lead/tin mixture is starting to solidify (as it cools). Basically follow the dotted line downwards (as the metal cools) and the metal become more and more solid until it hits line B-E where it is all solid (it doesn't all solidify at once, which is what the diagram is showing). The proportion of each of the constituents decides at what temperature these two things happen. Below the solidification line the temperature and composition will decide what structure the material forms (face centred cubic, body centred cubic etc). I'm more familiar with the iron state diagram and lead tin but the principles are the same. A represents the melting point of 100% lead E represents the temperature where solid lead/tin melts to liquid without going through a transition stage (like partial melting to form a paste). This only happens for lead/tin with the composition shown on the diagram, all others go through the paste stage. This point has a specific name but I can't remember it off hand. Googe `lead tin state diagram' Good luck.