[relknes]

Most partnerships have some set of meta-agreements about when a double is for takeout, penalty, support, cooperative, etc.
My partner and I use a pretty simple set. Doubles are for takeout unless:

1. partner and I have already agreed on a suit (double is for penalty)
2. someone has bid NT (double is for penalty)
3. three suits have been bid (snapdragon if partner bid, penalty if partner was silent)
4. you had a chance to make a takeout double of the suit earlier and didn't (for example, after (1♠)-P-(2♠)-P-(3♠)-X is penalty)

This is probably not optimal, but it is pretty easy and prevents misunderstandings. We are considering modifying a few of these agreements (#4 in particular may be better as takeout in the balancing seat when the opponents end in 2 of a suit, along with a double after 1m-(1M) or 1M-(2m) showing 4 in the unbid major rather than both unbid suits).

What are your partnership agreements?

[helene_t]

First of all, whatever the double means it must be consistent with the previous bidding. For example, if I fail to make a 1-level overcall in spades and subsequently double a spade contract, even if it is penalty according to our meta agreements it can never show such good spades that I should have bid them myself before. Another example: if we have found a fit and I have extra o/d justifying bidding one level higher then I should do so, even if partner doubles them ostensibly for penalty.

The main principle is that when partner's hand is sufficiently well described to transfer captainship to you, your double is penalty. Which auctions make partner's hand sufficiently well described? I don't have a complete list, but:
- Almost all jump bids (except for vague strong jumps such as jump responses to a take-out double, or a Polish 2♣ opening).
- Some notrump bids (we can make some specific exceptions but I probably prefer so many exceptions that it would be better to make t/o the default here)
- Most situations in which partner has made two shape-describing bids
- Conventional two-suited bids such as landy or michaels
- Converting an earlier take-out double to penalty
- Some low-level passes that carry very strong negative inference such as an opening followed by a pass after RHO's 1-level sandwich overcall.

"Delayed takeout doubles" (i.e. doubles which I could have made before) only applies if both of us have been passing throughout and only op to a certain level, I think 3♦.

If partner doubles for t/o then I can double the same suit for t/o up to a certain level (I think 4♦) but if they change suit or bid a natural nt then my doubles are always penalty.

If we have already found a fit, double can never be take-out but how penalty-oriented it is depends, again, on how well-described partner's hand is. Generally it just shows some extra values that will be useful in defense.

Doubles on natural 1NT bids are mostly t/o. 2NT always penalty (but we would use this very rarely since the normal strategy is to wait for them to raise to 3NT!). 3NT always penalty except for a 3NT response to a preempt.

Doubles on conventional non-GF raises are t/o but doubles on GF raises are lead directing.

Double on other conventional bids that show limit+ strength are lead directing. Doubles on conventional bids, that don't promise limit+ strength, just show general strength.

You also need agreements for forcing pass situations. I prefer to play double as optional if we have found a fit but t/o otherwise.