A detail of the equilibrium phase diagram of the copper-tin (Cu-Sn) system shows the different phases that may exist in the microstructure of such an alloy depending on the temperature (vertical axis), and the weight percentage of tin in the alloy (horizontal axis). In pure copper, Cu atoms are aligned in a face-centered cube: this is the alpha (α) phase (see fig. 139). The delta (δ) phase forms when there is more than ~20 wt% tin. This face-centered cubic phase is responsible for the specific acoustic properties of bells. The diagram also indicates melting temperatures depending on the alloy composition. For example, an alloy with 10% tin melts fully at ~1000°C, once the liquidus line is passed, as shown by the red dotted lines. It solidifies fully at ~830°C, once the solidus line is crossed.