Playing 5-card majors, we have to decide what to open on a balanced hand outside the range for 1NT. In the most widely-played systems, these hands are shared between the 1C and 1D openings. But an alternative approach is to put all these balanced hands into 1C. This is what we do in Siege (as described on Mike Bell's system page here). The result is that 1D promises an unbalanced hand (even 5-3-3-2 shapes with 5 diamonds are generally opened 1C).
We also open 1D on any hand with both minors, even if the club suit is longer. As well as being more descriptive (showing four cards with the first bid instead of two), opening 1D on these hands makes it easier to find a rebid. It also helps the 1C opening a little, since a 2D rebid is freed up for some alternative meaning.
In fact, Siege is far from the only system which uses this sort of 1D opening. There are other very similar short club systems, such as the one played by Welland/Fallenius which Siege was originally based on. And the same 1D opening can also be played as part of a strong club or multi-way club system. It is an essential part of Millennium Club, for example, and I like to use it in Polish Club as well.
The best thing about this 1D opening is that it promises a very suitable hand for playing in diamonds - much more so than a standard 1D opening which could often be weak and balanced. Our 1D opening can be raised almost as freely as a major-suit opening. In particular, a weakish hand with 4 diamonds and a 4-card major can normally afford to raise diamonds immediately rather than bidding the major, since if a major suit does exist it will not be as good as the diamond fit (except in the rare case that opener has a 4-4-4-1 hand). It is good to have several different ways of raising diamonds available, for example:
2D = weakish with 3 or 4 diamonds;
2NT = either a very weak raise, or a game-forcing raise;
3C = invitational raise;
3D = weakish with 4+ diamonds;
3M/4C = splinter.
The continuations after a 1D opening make use of the fact that opener will not have a balanced hand. After a 1H response (which is natural), we use the spare 1NT rebid to distinguish between the various types of minor two-suiters that opener might have:
1NT = both minors, with clubs at least as long as diamonds;
2C = both minors, with diamonds at least as long as clubs.
And so we can usually find our best fit when responder is giving preference.
Over a 1S response it is more useful to use the extra bid available to show hearts, solving the rebid problem that we have holding five diamonds and four hearts in standard methods. So we play:
1NT = both minors, either suit may be longer;
2C = hearts.
The advantage of using 1NT to show both minors, rather than the natural 2C, is so that when responder is (say) 2-3 in the minors with 8 or 9 high-card points he can rebid 2C, keeping the auction alive in case opener has a strong hand. In standard methods after 1D:1S,2C you might give false preference to 2D on this sort of hand, but this is not so attractive in Siege since opener could have only four diamonds.
We also have a 2NT rebid available, and can use this to help solve some difficult problems after a 1-of-a-major reponse:
2NT = Good hand with 6+ diamonds, denies 3-card support for partner's major unless intending to force to game;
3D = Good hand with 6+ diamonds and 3-card support for partner's major (but not forcing).
Opener's raise of a major-suit response to the two-level will frequently be made with only 3-card support. For this reason, when holding a minimum hand with 3=3=5=2 shape we can choose to open 1D, intending to raise a major-suit response. In principle this is the only time that 1D can be opened on a balanced hand in Siege.