This axiom was invoked in discussion on GreatDesign, and got me thinking.  What the heck is this clause supposed to mean?  I found a reference in Bartlett's familiar quotations to a quote by Robert Burton, tracking the axiom to around the early 17th century.  I also found an article at http://devworld.apple.com/technotes/tn/tn1067.html which claims that this is better read as "The exceptions ''test'' the rule".  Thus, I can identify three (now four) possible uses:

	1. A speaker proposes a rule of thumb, and is challenged with an apparent counter-example.  As a flippant response, he simply quotes "The exceptions prove the rule."  This seems to be a common current response, but I imagine this was driven by a deeper, earlier meaning.

	2. The meaning given in the devworld article above is true.  Thus, the axiom is advice that to see if a rule holds, it is best to closely examine and test apparent contradictions.

	3. The meaning I seem to find most intriguing holds, which is this.  The axiom encourages us to remember the difference between "rules", which state useful properties of the world around us, and "tautologies", which state potentially uninteresting (in a certain context) logical properties of language.  For example, if someone stated "Good software teams have a software architect", and would not admit any exceptions, you might be tempted to conclude that they were saying that they only apply the word "good" to teams with a software architect--this would be a tautology.  On the other hand, if they said "Good software teams have a software architect, except, of course, the MegaFastCorp Inventory team," then it would seem that they are stating what they believe to be a valid conclusion from the data presented them, in which the complying data far outweighs the few known exceptions.  

	4. RobertDiFalco's excellent commentary below provides another good meaning, from the legal maxim: ''the exception proves the rule in cases not excepted''.  Thus reading "Contest not valid in Utah, Vermont, or Florida" can make good strides toward proving that the contest ''is'' valid in Ohio, since exceptions to the rule ''are'' listed, and Ohio is not one of them (Ohio is a "case not excepted").  Very nice.

-- DavidSaff

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Numbers 2 and 3 seem reasonable. I find that number 1 sounds like a cop out. If I had to choose a single meaning, I'd probably choose number 2. The full phrase is ''the exception proves the rule in cases not excepted'' is a legal maxim. In the DevilsDictionary, Ambrose Bierce says:
	 :	"The exception proves the rule" is an expression constantly upon the lips of the ignorant, who parrot it from one another with never a thought of its absurdity. In the Latin, ''Exceptio probat regulam'' means that '''the exception ''tests'' the rule''', puts it to the proof, ''not confirms it''. The malefactor who drew the meaning from this excellent dictum and substituted a contrary one of his own exerted an evil power which appears to be immortal."

''--RobertDiFalco''

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Prove as in proofread, or test new cars at a ''proving'' track. -- RobertField

Exception as in ''source-code'' and the rule as in ''design'', or was that the binaries. ;) --rad

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My understanding of the historical meaning of this phrase
(bearing in mind that common usage now bears no relationship
to the history) is this:

	 :	An explicit exception can be taken as confirmation of the existence of an ''implicit'' or unwritten rule.

For example, seeing the notice "Special leave is given for 
men to be out of barracks tonight till 11:00 p.m." would 
strongly imply that the ordinary rule requires men to be
back in the barracks earlier than 11.  And in a legal 
situation, this might be sufficient proof even if the rule
were not explicitly recorded anywhere.

(I think this is what RobertDiFalco means in his contribution above,
but the quote from Ambrose Bierce seems to be saying something different.)

I first learned this interpretation from my colleague 
MikeRieser, and I've found confirmation of it in the 
following two places:

	* http://www.alt-usage-english.org/excerpts/fxtheexc.html
	* http://www.straightdope.com/classics/a3_201.html

The second link starts out espousing some of the more 
common views, but comes 'round about halfway down in 
response to reader feedback and more research.  It also 
contains the interesting story about the origin of the 
phrase, taken from the writings of Cicero.
--GlennVanderburg