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Using "Either/Or" Limitations
Some concepts call for an "either/or" limitation. For example, a creature with Damage Resistance may have thick skin and an armored carapace on its belly (either Tough Skin or Partial, Vitals Only). Or the use of one's Luck may have to be declared in advance for everything except defensive rolls (either Active or Defensive). The rules do make these combinations possible, though it isn't immediately obvious that they're supported, much less how to implement them.
Fortunately, this doesn't even require a house rule. The guidelines for doing so are simple and derived 100% from the Basic Set:
When an ability is designed so it can be affected by either one limitation or an alternate limitation, multiply the percentage values of both limitations together to arrive at a final value.
For example, a pulp superhero has a knockout gas spray, bought as a Fatigue Attack with various modifiers. Because it's pheromone-based, only females get a HT roll to resist. The attack therefore has "Either Accessibility, Males Only (-20%) or Resistible, HT (-30%)" as a limitation. The player multiplies the two values together (0.20 x 0.30) for a limitation value of -6%. His ability is much more powerful than if everyone got a resistance roll, and thus costs more than if he'd just taken Resistible normally.
If either limitation somehow exceeds -80%, reduce it to -80% for this calculation.
Anyone who's read the box on Fantasy, p. 130 (Behind the Curtain: Cost of Divided and Restructurable Magery) has seen a glimpse of this already, albeit a rather complicated one. It's just an upside-down way of implementing the optional Limited Enhancements rule in the box on p. B111. If a power has a limitation worth -X%, then you could think of not having that limitation as a +X% enhancement. You then limit the enhancement (with the second limitation).
Let's take the sleeping gas example again. If the pulp hero were to take "Resistable, HT" as a limitation, that would be -30%. Now, an enhancement that removed that limitation, a "Not Resistable" enhancement, would be +30%, to cancel it out. Since we only want males to be affected without a resistance roll, we need to use the Limited Enhancements rule to limit the "Not Resistable" with the -20% limitation "Accessibility: Males Only". Taking 20% off of a +30% enhancement leaves +24%, which is the value of the limited enhancement "Not Resistable (Males Only)". The net result? A -30% limitation and a +24% limited enhancement, for a total -6% limitation. Since the math works out the same every time (math is good like that), we can just use the simpler method of multiplying the two instead.
The "-80% rule" is just a rewording of the rule that a "Limited Enhancement" cannot be reduced below 1/5 of its normal value. Since each limitation can be seen as limiting the other, neither one can be greater than -80% for the purpose of this rule. Since there are no official limitations greater than this (and likely never will be), it shouldn't come up much.
Limiting Partial Advantages
This rule also works for putting a limitation on only some of the levels of a levelled advantage. For example, a psi might have TK 30, but can only use one-third of it except in direct sunlight. This can be bought as two different advantages, of course:
TK 10 
TK +20 (Only in direct sunlight, -30%) 
And that's often the easiest way. But sometimes, especially with long, complex advantages, you don't want to write it out multiple times. In that case, just multiply the limitation value by the percentage (or fraction) of levels that you're limiting. For example, since you're limiting two-thirds of the TK (in levels), the limitation is worth two-thirds as much:
TK 30 (One-third power except in direct sunlight, -20%) 
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