If the Monster & Treasure Assortments had got more play…

—“What’s your Hack-9?”
—“What’s that?”
—“You know, your attack level—what do you need to Hit Armor Class 9?”

“AL = level of attack by monster as expressed by the monster’s base number to score a hit on an unarmored opponent (armor class 9)”—Monster & Treasure Assortments (TSR Hobbies, 1977)