|Double||0, 2 or 4 aces|
|Pass||1 or 3 aces|
If playing Roman Key Card Blackwood, a double would show 0, 2 or 4 keycards, while a pass would promise 1, 3 or 5 keycards.
If partner bids 4NT followed by 5NT, he is making a Blackwood query for kings. All other rebids by partner are natural signoffs.
DEPO responses are vaguer than DOPI, but can be played over a higher level of interference. For example, if the opponents sacrifice at the 6-level, there may be insufficient space to make DOPI responses and still land in a safe contract. DEPO allows you to double or pass without raising the bidding level any higher.
See also ROPI when the opponents double 4NT.
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