Play ALL transfers over an 18-19 2N rebid. There is virtually no hand you can't show. Here, after 3♣-3♦, responder should be showing 6+ good ♦ after not passing 3♦. With 5+♦ & 4M, South just transfers to the 4-card major. Playing ALL transfers also lets you play in 3♦ if needed. There are only two transfers that opener can decline: 1.) 1x-1y // 2N-3♠, the transfer to 4♣ maybe declined. However, if responder still bids 4♣, that is to play. 2.) 1x-1♥ // 2N-3♥. Since responder can have only 4♥ along with 4♠, opener should only bid 3♠ with 4+♠. Else, opener bids 3N. After this discussion, there is probably a third time the transfer might be declined. Namely, 1♣-1♦ // 2N-(3♦ or 3♥) where opener only accepts the transfer to the major with 4-card support. I just never thought about this sequence since we often by-pass 1♦ to bid our major unless we have EXTRAS. [How to show (5+♥ & 4♠) OR (5+♠ & 4+♥) are exercises left to the reader. :P ]