Lessons numbered with a whole number (1, 2, 3, etc.) are the main lessons. You should read them in order. Lessons whose number has a decimal point (2.1, 3.2, etc.) are optional lessons. You don't have to read them at all. For instance, you should read Lesson 1 and Lesson 2 before Lesson 2.1. But you can continue on to Lesson 3 without reading 2.1, if you like.
