When engaged in melee combat, you will always hit on a roll of 20, whatever
the AC of your opponent. This will also score a "threat," meaning that you
have a chance of scoring a critical hit. Under certain conditions, your
'threat range' can be extended to include lower dice rolls (see Improved
Critical, for example). In any case, when you score a threat, you then check
against the combat skill you are employing; if successful, your hit becomes a
critical hit and the damage is multiplied by the factor indicated on your
score sheet.
* Certain creatures (for example, dummies, oozes, and slimes), have no
central nervous system or critical parts, and are not vulnerable to critical
hits.