Last week I saw this tweet by @lisasolomon:
@KonciseD Agreement also uses "shall not…unless" construction instead of "may…only if." Keep fighting the good fight, Ken!
— Lisa Solomon (@lisasolomon) July 31, 2014
Acme shall not sell the Shares unless Widgetco consents.
Acme may sell the Shares only if Widgetco consents.
My Twitter interlocutors all favor option two (language of limited discretion) over option one (language of prohibition with an exception), on the grounds that it’s easier to understand concepts that are expressed in the positive rather than the negative.
Although I haven’t yet had a chance to research that notion (Semantics and Comprehension, anyone?), it sounds sensible. But I wondered whether desirability of the action in question is a factor:
Acme shall not destroy the Equipment unless Widgetco consents.
Acme may destroy the Equipment only if Widgetco consents.
I think there’s something to be said for stating as a prohibition that which you really don’t want to happen. But I invite all you semantics ninjas to wade in on that.
I should also mention the ambiguity inherent in may … only. Consider the sentence Widgetco may sell only the 1965 Ford Mustang. Presumably the intended meaning is that the only vehicle that Widgetco is permitted to sell is the vehicle specified. But it could also mean that Widgetco may elect to sell only that car but would also be free to sell other cars instead of or in additional to the Ford Mustang, or not sell any cars. In other contexts, the more natural meaning is analogous to the latter meaning. (See MSCD 3.155–59.)
Those alternative meanings present themselves even if instead of being part of the subject only introduces a conditional clause, as in Acme may sell the Shares only if Widgetco consents. But the second possible meaning (consistent with the logic outlined in the previous paragraph) is so strained that I wouldn’t say that the alternative meanings give rise to ambiguity. So in this context, the ambiguity of may … only isn’t a factor.
[Updated August 6, 2014: Thank you all for a very enlightening exchange. I have made my decision; let's call it "The Decision," LeBron James-style.
Henceforth, I will aim to use—cue drum roll—shall not ... unless.
It boiled down to choosing the lesser of two evils. One of the evils is the modest tax on comprehension that comes with expressing an idea in the negative rather than the positive. The other evil is the alternative meanings that come from using language of limited discretion. The expectation of relevance suggests that if Acme may destroy the Equipment only if Widgetco consents, then Acme may not otherwise destroy the Equipment. But as my linguistics buddies are fond of saying, that's an implicature—it's not the only meaning; see A. Wright Burke's comment.
The odds of the alternative meanings associated with the may ... only if version creating a problem are infinitessimal. But I prefer to train myself to spot ambiguity and eliminate it wherever possible, to increase the odds of my catching those instances that might well lead to dispute.
This isn't my final word; I'll have to refine my analysis for purposes of MSCD4, and I wouldn't be astonished if I think of some new angles. Meanwhile, I expect proponents of may ... only if to start rioting, flipping cars, and stuffing a lone cardboard-cutout image of me head-first in the nearest trash can.]