Think about what bid would give your partner the most information about your hand.

NT Opening and Staymen  by South.

West - tell your partner what to lead with "double".  When you double an artificial bid you are telling your to lead that suit.

North will continue the bidding to get to 4 Spades.

If you just bid Clubs, North may back out and plan to "set" you knowing NS have at least 23 HCP. 

Tell your partner you have 20 - 21 HCP.  You don't need a stopper in each suit.  You may get caught in an unfortunate situation, but start with the best bid your can make.

With both Majors south bids Staymen.

With the Spades response south bids 4 Spades.  The 2 NT bid limited Norths hand so no reason to look for more.

If North had opened 1 D  you may not get to games with South having just 8 points.

North opens 1 Spade.

East overcalls 2 Hearts

South - Take North to game.  You have 5 Spades so know your side has at least 10 Spades.  Also you have the A of Hearts so first round control of opponents suit.  Bidding 3 Spades partner may not go to game if a minimum opening.

North opens 1 NT.  Ease does not have a bid so passes.  After South passes, West can double.  East didn't bid but South did not transfer or use Staymen.  East must have something.

East bids his best suit and West passes.

North opens 1 Club and East overcalls 1 spade.  South passes.

West doesn't have 3 Spades but does have 11 points.  If west passes, North may come in with a Double making his partner bid.  Bid 1 NT.  Nothing special and East will pass.

North opens 1 Spade.  East overcalls 2 Clubs.  South passes.  West has 4 Clubs and East has shown 5 so bid 3 Clubs.  North bids 3 spades showing 6.  East bids 4 Clubs.