I thought it time to hoist out of the comments a discussion of obligation versus duty. Here’s what reader AWB said in this comment:
By the way, any time is a good time to reconsider your preference of “obligation” (ten letters, four syllables) over “duty” (four letters, two syllables). They’re equally Latinate, but “duty” is easier to chew and has fewer calories.
Here’s what reader Westmorlandia said in response:
I think “duty” is used more commonly to refer to fiduciary duties, duties of care and so on—non-contractual law that requires certain standards of behaviour rather than specific actions. I don’t often see those called “obligations”.
I also very rarely see contractual obligations referred to as duties.
I suspect there was once a clearer linguistic distinction than there now is, though this is just a hunch. The conceptual distinction is very useful in law, and I like the idea that the language can cater for it. That said, even if this were the case, obviously few people are aware of the distinction or use it consistently (at least consciously). And it may not quite be as simple as that. But I am fairly sure that the terms are not actually used synonymously, even in general parlance.
And here’s what Garner’s Dictionary of Legal Usage says:
obligation; duty. Broadly speaking, the words are synonymous in referring to what a person is required to do or refrain from doing—or for the performance or nonperformance of which the person is responsible. But there are connotative nuances. An obligation is normally an immediate requirement with a specific reference <his child-support obligations> <Burundi’s obligations under the treaty>. …
A duty may involve legal compulsion and immediacy, but the word carries an overlay of a moral or ethical imperative <parental duties> <fiduciary duties>. More specifically, duty = (1) that which one is required to do or refrain from doing, esp. as occupant of some position, role, or office; or (2) any one of a complex of rights and standards of care imposed by a legal relationship. Sense 2 appears primarily in tort law, in which writers use duty only to mean that there could be liability.
So Westmorlandia and Garner are basically of the same view. And now that I’ve gotten around to thinking about it, I share that view. That’s why I refer to the implied duty of good faith but refer to a party’s obligations under a contract.