Yeah, we'll definitely discuss it at length - I told him about the stopperless 1NT bid, we agreed it sounds reasonable, and then during last evening's session he made the same negative double (auction was 1♣-(p)-1♦-(1♥)-x) this time showing a three card spade suit! So yeah, we clearly should work out what double means over opponents' interference :) [edit] As far as when to double and when to overcall 1NT, we're playing in MP tournaments so if 1NT looks feasible, that's probably where we want to be. Of course, now that we allow a stopperless NT, it's probably better to always show four hearts if you have a non-positional stopper, I think.