NEGATIVE INFERENCE

"You see, men" said Capt Mainwaring firmly to the assembled platoon, "when you make a bid, partner can infer that you have such and such, depending on your bid."
"Such as such as what?" asked Pike.
"Such as whatever you bid, stupid boy."
"Such as what have I bid?"
The Captain paused while untold vile and disgusting epithets fought in his mind to be heard. When he had controlled himself he again took breath...
""Are you alright Captain," asked Private Godfrey, "Only you went a bit red then."
"I'm fine thank you Godfrey. Now where was I?"
"You were about to tell Pike what he'd jolly well bid." Said Lance Corporal Jones
"But I haven't bid anything yet. That's the whole point," said Pike smugly.
"It was," said the Captain in a voice of pure cold steel, "An hypothetical bid..."
"I don't think we've heard about them!" said Private Frazer.
"Perhaps if the Captain could just continue...?"
"Good idea, Wilson. I shall do just that. Now, chaps, if you open 1C or 1D and partner responds 1NT you will know, or can positively infer, that they have exactly 6 - 9 HCP. No more, no less. But there is also a Negative Inference that you are entitled to draw: to wit - that they also do NOT hold a 4 card or better Major suit. There is therefore no earthly point trying to find a 4-4 major suit fit later in the auction as you already know this to be an impossibility. Which is the reason why if you, as responder, DO hold a 4 card major then you MUST bid it, otherwise partner will not be able to evaluate the hand properly. It also follows that when partner opens 1 of a suit (ie does not open 1NT!) you can make the reasonable Negative Inference that they do not have a balanced 12 - 14 count. Just be sure your inferences are appropriate to your holding!"
"Blimey!" said Walker, "I met a land-girl the other night and she was like that: negative to what I was holding!"