LEARNING CENTER

No, this is not a support double. Remember, support doubles can only be made by the opening bidder.

P.S. This particular double is rare and not worth studying, but for the curious:

this double is by the advancer and it's meaning depends on partnership agreement, but a common agreement says that it shows the fourth suit with tolerance (usually a doubleton) in the overcaller's suit. 

Yes. This is a support double. Opener is showing exactly three hearts and could have extra values or a minimum opening bid. 

2. You would love to have a fifth spade as you know that you are heading for a 4-3 fit, but nothing else looks any better. 2 here does not promise that you have a real fit. 

Pass. Double would tend to promise three hearts, but more importantly it would say "I still want to compete to the 3-level opposite your 6 points". We don't have enough value to do that. The same reasoning prevents us from bidding 2NT as 2NT in this situation would promise 18-19 points. Pass and let partner do something intelligent. 

4. You have enough values to be in game and you know you have an 8-card fit. In this case partner didn't make a support double, but a support redouble. This works just the same as support doubles (the opponents have interfered and partner can show 3-card support below two of our suit). Again, the key is remembering that you have this tool available. 

2. This time, we might have a better fit in partner's suit and we can find out on the 2-level. This bid is not forward going. It simply says "Hey, partner, maybe diamonds is better than spades because I do not have five spades."