The game includes a number of Rules Options, allowing groups to customise the game to their taste. The main option (Armour Class system) is listed here. Other optional rules are noted in various sections of the game—these increase the game’s mechanical richness and complexity. Players should ask the referee which options are in use.
Beginners: Are advised to use ascending AC (described below) and variable weapon damage. Other optional rules can be introduced as players grow more familiar with the game.

The game supports both the descending AC system used in 1970s–1990s editions of D&D and the ascending AC system used in later editions. The AC system used determines how attack rolls are resolved.

AC values: Lower is better (e.g. unarmoured is AC 9, chain mail is AC 5).
AC modifiers: Bonuses decrease AC and penalties increase it.
Attack rolls: The target number for an attack roll is looked up in the attack matrix, with a THAC0 (“to hit AC 0”) value indicating the row to use.

AC values: Higher is better (e.g. unarmoured is AC 10, chain mail is AC 14).
AC modifiers: Bonuses increase AC and penalties decrease it.
Attack rolls: The target number for an attack roll is the defender’s AC, with an attack bonus added to the attack roll.

AC scores use a dual format, with the descending AC listed first, then the equivalent ascending AC in square brackets. e.g. AC 5 [14]—an AC of 5 if using descending AC or 14 if using ascending AC.

Attack values use a dual format, with the THAC0 (“to hit AC 0”) value for descending AC listed first, then the equivalent attack bonus for ascending AC in square brackets. e.g. THAC0 17 [+2]—the attacker uses attack matrix row 17 if using descending AC or an attack bonus of +2 if using ascending AC.