The Third Law of Thermodynamics

However, I've slipped something past you. I've given numbers for entropy without saying where they come from. To get entropies of a substance, we need to introduce our fourth, and final law of thermodynamics.¹

Third Law: The limiting value of the entropy of a system can be taken as zero as the absolute value of temperature approaches zero.

There is really not much interesting to say about this now. Most applications of the the third law appear in statistical mechanics.²Except, notice that it allows us to calculate the absolute value of entropy by integrating $\int_{T=0}^{\mbox{any state}} \frac{d q_{\mbox{rev}}}{T}$