The One Diamond Opening Bid

Opening one diamond with 11-15 HCP and perhaps only a singleton in the suit might be the weakest part of Nannie.  The best way to treat it is as a natural bid but semiforcing, but what does semiforcing mean?  Hmmm... well, it's easy to pass holding no points and four or more diamonds, but what If responder is short in diamonds?  If partner has opened with a small doubleton in that suit the opponents may let her play there.  That can't be good.  So, regardless of how few points responder may have, she should bid unless she has at least four diamonds.

Bidding with no points means we will probably get too high, but at least we are more likely to have a fit and less of a disaster.  Occasionally we will even steal the hand from the opponents.

Passing 1 implies at least four diamonds.

Responding 1 or 1 does not promise 6 HCP.

It can be demoralizing playing in a 2-2 diamond fit, especially when you have a 4-4 fit in a major.  Let's try to avoid it.

What I am about to advocate may not sit well with you, but it works.
Suppose you hold this hand:

                  Axx
                  xxx
                  xx
                  xxxxx

You can't pass and you have no bid.  What to do?  May I suggest you bid 1 and not 1?  Even if you had four spades this would be the right response.
Here's the reason - If partner has four hearts she will raise you and you can pass, playing in a 4-3 fit.  It can't be worse than playing in diamonds.  But if instead she rebids 1 you can pass and perhaps play at the one level. And of course you would pass a rebid of 1NT.  (And wouldn't it be great if partner were to rebid 2?)

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INVERTED RAISES

Responder Raises to 3
A jump raise in diamonds is weak!  It also denies a four card major and should have a minimum of five diamonds.  Mostly its purpose is to be preemptive and keep the opponents off balance.

Responder Raises to 2
With 12+ HCP and no four card major an immediate raise to two diamonds promises at least four diamonds and asks the question,
     "Can we can play in a notrump contract, partner?"

A longer discussion on the Inverted Diamond Raise can be found here.