Puzzle #57: Alphabet Sets

The letters of the alphabet have been divided into three sets. What are the set membership rules?

Set 1: A E F H I K L M N T V W X Y Z

Set 2: B

Set 3: C D G J O P Q R S U

Submit your answer to Gene Wirchenko <genew@telus.net>. Your answer should be in the form of a proof. That means to show how your answer must be correct. The deadline is Wednesday, July 9, 2014 at noon Pacific Time. I will post the answer shortly after.