Another major source of carbon emission is the need to heat and cool buildings. Here, too, urban density makes a difference. When we live in smaller spaces, it takes less energy to make them comfortable. Imagine a stand-alone building that is home to a single couple. Now imagine an identical building subdivided into a triplex, with a couple living in each unit. On a muggy Philadelphia summer day, the couple in the first building need just about as much energy to keep their building cool as the six people living in the triplex. Multiply this effect across an entire city, and the savings are substantial.