Mix two metals and something strange happens: at one exact recipe the blend melts at a temperature LOWER than either pure metal on its own. That’s why electrical solder (tin + lead) melts at a friendly 183°C when tin melts at 232 and lead at 327 — the two temper each other. Slide the composition and find the valley.
A eutectic system has a composition at which the liquid freezes at a single, MINIMUM temperature — below the melting point of either pure component. For tin–lead: pure Pb melts at 327°C, pure Sn at 232°C, but the eutectic at ~62% Sn melts at just 183°C, and freezes to a fine two-phase microstructure all at once (no mushy range). The two liquidus lines slope down from each pure end and meet at that eutectic point. A fail-loud self-check throws unless the lowest point on the melting curve is below both pure metals. ◆ real materials science, node-verified.
An idealised two-line liquidus meeting at the eutectic (the classic Pb–Sn diagram); real alloys add solid solubility and intermetallics — the below-both minimum at a fixed composition is the exact, defining feature.