One of the basics of Bridge that gets missed time & time again is the relationship between a penalty double or conversion & partner's suit. A penalty double or conversion means I am very short in your suit partner so I do not detract from the defensive potenials of your HCP's. The auction is a misfit meaning I do not fit your suit when I double , convert for penalty or sometimes just by not competing. This is a Bridge Law !!

An experienced player held this hand all vul ♠Ax ♥J10 ♦AQ ♣Q10987xx , her RHO opened 1♠ so she overcalled 2♣ & LHO bid 2♠ . Partner bid 3♦ & RHO bid 4♠ so what do you do ? 6 of your HCP's are in partner's suit so a double is eliminated automatically. Expert players would bid 4NT which is an invitation to bid 5♣ with diamond tolerance. Our herorine doubled & a double game swing occured . 5♦X is cold for +750 & partner with a void in clubs left in your penalty double for -590 your way.

A player held this hand ♠void ♥AJxx ♦AKJx ♣Kxxxx & opened 1♦ which his LHO doubled. I passed & RHO bid 1NT so you competed to 2♣ . This was passed by partner so her diamond length is now known. Your RHO bids 2♠ so you show your strength by doubling . Partner converts for penalty & this board hits ♠AKx ♥KQ10x ♦xxx ♣109x . How do you defend ? Applying patterns in the trump suit is your first duty. Partner has 6♠ !! You know partner only has 3♣ ( NT bidder has a doubleton ) & she showed preference by passing so she has 2 or less diamonds. You continue diamonds & partner discards a heart. You continue diamonds & partner ruffs & returns the ♠Q . Declarer is on the board so he tries the club finesse. This loses so you return a diamond which partner ruffs & returns the ♠J . Again declarer is on the dummy so he leads a club to his Ace & a heart towards the dummy which you duck . He ruffs a club so he gets 3♠ tricks , 1♣ , 1♥ for down 3 doubled.