|See the gray cat on the left? Yes, that's a backstab. To the face.|
When making a melee attack, characters receive a +1 bonus if their opponent is threatened by another enemy character or creature on its opposite border or opposite corner.
When in doubt whether two characters flank an opponent, trace an imaginary line between the two attackers' centers. If the line passes directly through opposite borders of the opponent's space (including corners of those borders), then the opponent is flanked.
Characters who have the ability to backstab, or another similar ability, can attempt to backstab if they flank their opponent. The opponent receives a saving throw to take half damage if the opponent is not surprised and if the backstabbing character is not hidden or invisible by some means.
Note: Why would I add this to an "old rules" game where so much of the action takes place in the theater of the mind? Well, I like miniatures, for one, and for two, this totally solves the dilemma of "When can I backstab?" If you're like me, you barely used it in 2E and BECMI since the rules simply said "behind the target" or something similar. Plus, this gives the thief a bit of added punch. They don't have the hit points to duke it out, so if they dare get that close to an enemy they'd better hit hard once and scatter.