In one partnership, we play the following (expressed from opener's POV following RHO action; 1♣ = 16+ any and responder's bids shows 5+ in suit and GF unless noted):
1) If we haven't agreed on a suit, XX is to play (example: 1♣* - 2♠ (any 4441, GF) - (X) - XX)
2) A direct double is almost never purely for penalty and is generally takeout oriented. The general rule is that Pass requests a X, direct bid tends to show a single suited hand and pull after the X is either two places to play or support with slam interest.
Specifically:
- If responder has shown a suit and they have bid past the cheapest level in that suit, X is an optional raise (1♣* - 1♥ - (4♠) - X)
- Raise following an initial pass is slam interest if they bid past the cheapest game level (direct raise is fit with no interest in defending: 1♣* - 1♥ - (4♠) - P - X - 5♥* (expressing slam interest))
- If they are one below the cheapest game level for the suit, X=good raise of the suit (1♣* - 1♥ - (3♠) - X)
For example, an interesting application of the above might be when opener holds 5-5 in the blacks and the bidding goes:
1♣ - 1♥ (5+ GF) - (3♦)
Now, opener can pass and pull 3♦ to show the 5-5 black hand (contrast this with having to bid 3♠ with both with a single suited hand or the hand with the blacks). Naturally, vulnerability considerations may influence whether opener bids 3♠ anyway, but it's an option.
Thoughts?