A word about Weak Jump Overcalls. Say an opponent opens 1 ♣. If you jump to 2 ♥ , you’re showing a hand which would have opened (a Weak) 2 ♥ ; if you jump to 3 ♥, you’re showing a hand which would have opened 3 ♥. Simple and highly effective.

Back to responding to the level of the fit. Say partner opens (or indeed makes a Weak Jump overcall of) 3 ♦ . Respond with these:

With the first, bid 5 ♦ . There’s an 11-card diamond fit, so bid for 11 tricks. You would do this non-vulnerable or vulnerable. But if your shape was duller, making two of your clubs into hearts, you’d be wise to settle for 4 ♦ if vulnerable.

With the second, raise to 4 ♦ , the (ten-card) level of the fit. Your opponents will now have to guess what to do at an uncomfortably high level.

With the third, you need to use your nous, because of the defensive nature of your hand together with the flat shape. 4 ♦(doubled), the level of the fit, could lose a lot of tricks; and there’s no guarantee the opponents are making game. The wise action is no action. Pass. The level of the fit is a guide not a gospel.