Advanced Combat and Encumbrance
Advanced Combat and Encumbrance (or ACE) is a system designed by adventure author Sam Ruby for simulating weapons, armor, and combat in a more sophisticated and realistic manner than the default Eamon system. Ruby included an early form of the system in The Boy and the Bard (1989) and a more complete and well-documented version of it in Sanctuary (1990).
Ruby included with Sanctuary a lengthy set of "combat notes" that describe the principles of ACE and the particulars of how it's implemented. The full text of Ruby's notes is available here: Source:Sanctuary notes.