The law of sines is well-known, but its proof is not. It turns out that the expressions in the law of sines can be proven to all be equal to twice the circumradius, which not only proves the law of sines but extends it. We prove this result here, which says that $$\frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C} = 2R.$$