Third, Keybr.com lets you introduce as few keys as possible to the lesson, adding more keys automatically when it decides that you are proficient at the current level. When you only start learning it generates lessons with words from a very small alphabet of the most frequent letters. When your typing speed for every key in that alphabet reaches certain threshold, the algorithm adds the next most frequent letter to the alphabet. And so on, until the next letter. This way you will learn the most frequent letters first, and the least frequent ones later.