[GarthB]

1. Deceptive: The pitcher is attempting, by design and action, to "illegally" deceive or "fool" the baserunner. The defense is attempting to gain an unfair advantage over the offense.
Examples: a,g,i, j, k and m.
2. Mechanical: The pitcher isn't really trying to deceive anyone. He simply isn't proceeding in his mechanics as section 8.01 would have him. Examples: c, e, f.
3. Penal: Certain actions by a pitcher, so says the rules committee, are illegal or improper and the appropriate penalty, therefore, is a balk. Examples: b,d,h,l as well as those under Rule 2.00.

The above is from an article I wrote for the paid side of this site back in 2000. It is based on both historical research and a conversation I had with Jim Evans.

There will never be 100% agreement as to which category every balk belongs, and there is nothing wrong some debate. As Carl Childress noted:

"In the long run it doesn't matter whether 8.05(a) is deceptive or penal. The main reason to categorize balks is to impress on umpires that more than one type of infraction deserves the call. We've all heard the umpire say: "Well, he didn't try to deceive anybody." Discussing penal and mechanical balks is a way to move umpire thinking away from that elementary notion."