South, believing that his partner held genuine ♠ support - three cards - invited game with 3♠. This was an error. North was merely giving preference back to South’s first choice trump suit, holding equal length
.
3♠ was not a happy contract. West led ♣K then switched accurately to ♠4, trying to remove dummy’s trumps so that declarer could not trump ♥s. Declarer won East’s ♠Q with ♠K and led ♦4. West rose with ♦A, cashed ♣Q and led ♣3, to dummy’s ♣9, East’s ♣10 and declarer trumped. He now cashed ♠A and led a third ♠ hoping for an even split. Not so - West won ♠9, cashed ♠J, drawing declarer’s last trump (East discarding ♦5 and ♦J), then switched to ♥J. This ran to declarer’s ♥Q; he cashed ♥A and exited with ♥4. East won ♥10, cashed ♥K and his last card was ♣A. Declarer had scored just three trump tricks and ♥AQ - down four.

ANDREW’S TIP: Do not confuse genuine support with mere preference.