The 37° line is the latitude, from Sol's first study, below which a building's sunniest side switches from south to north for part of the summer.

The study sampled the sun's position every four minutes of every day of the year for 88 cities, on a vertical pane facing each of eight points, clear sky and open horizon. South glass is the sunniest in 74 cities and north glass in 13; between them the switch appears wherever midsummer sunrise and sunset stand far enough north of east and west, which is a matter of latitude alone.

Nearer the equator than that, the early-summer sun hangs in the north-east and north-west long enough to beat the south face — 15 days a year in Las Vegas, 191 in Nairobi, never in Chicago.