The Myler Level of any horse is based on his behaviour and experience. It's a gratuated system, meaning that as the horse learns, it earns more tongue relief. As you go up the Myler Levels, the correlating mouthpiece designs offer more tongue pressure. When the horse resists the bit, go softer not harsher.
Myler mouthpiece designs correspond to the Myler Level of a horse's behaviour and experience. The mouthpiece Levels start at Level 1, which apply maximum tongue pressure and go up to Level 3, which apply minimum tongue pressure and offer the most tongue relief. Some horses may never use a Level Three mouthpiece and that's ok. It's not about reaching the next Level. It's about finding the bit for your horse that gives him as much tongue relief as he can handle and give you the control and communication you want.
Important Note: Myler levels focus on the degree of tongue pressure or tongue relief offered by a mouthpiece. Be aware that other bit manufactures may have their own categories or levels based on different criteria, not to be confused with the Myler Levels.