Tramticket, on 2018-January-10, 09:44, said:
But if South opened 1♥ instead of passing and I make a standard take-out double I might have only thee spades and partner may have the same dilemma with 3-3-3-4, 3-4-2-4, or some similar shape.
Why does it suggest a three-card spade suit?
1) Yes, but you are only at the one level, and if North bids, partner can pass. (Added- yes, you might be at the one level here, too, but a 2 club bid avoids that potential pitfall).
2) If you had a 4 card
♠ suit, you would certainly double (or bid some number of
♠), so 4-1-4-4, 4-4-3-2, 4-0-5-4 and 4-1-5-3 are ruled out by a 2
♣ rebid.