 They created a method called Billings. Quickly following that, one of their students that they taught Billings method segued off of it and created another method called the Creighton method. And then around that same time in the 50s and 60s, a symptom thermal method came out, which is, as you mentioned, the thermometer. And then more recently, and the method that I teach is called the Marquette method. So that's an additional kind of a fourth way of tracking.