It often happens that the play of one suit is determined by the result of another. It is obvious that in such cases the determining suit must be handled first.
Let us start with a simple example:
South plays Six No Trumps and West leads the ten of diamonds.
There exists a perfect safety play for four tricks in the spade suit. But if the heart finesse does not succeed we shall need five tricks in spades, so we shall have to finesse the King. The play has to proceed like this: 1) 
A; 2) heart finesse (succeeds); 3) Q_4) small spade from hand. If both opponents follow suit, We can’t’lose more than one trick. If one of them shows out we continue accordingly. If at trick . No. 2 the heart finesse fails, we – have to try the spade finesse.
Our second hand is a bit more complicated:
Against South’s Six Diamonds, West leads the Queen of clubs.
By proper. play the contract is foolproof. Only a 4-1 or 5-0 heart break can mean any trouble. We can easily detect- without losing the tempo- which opponent holds four or more hearts, simply by cashing the two top honours.
If East has the singleton heart we cash the club and finesse the Queen of spades. Any return will establish two discards.
If West has the singleton heart we cash Ace of clubs and Ace of spades, and then run the Queen of spades, discarding a loser if East does not cover. Again, any play by West will establish a discard. If East covers the Queen of spades we ruff, enter dummy with a trump and lead the 10 of spades, repeating the same loser-?on-loser play.
A final hand illustrates an additional principle, that of Taking all Chances:
West leads the Jack of spades agai nst South’s contract of Five Diamonds. Trumps-break 3- 2. What is the best continuation ?
If only one trick need be lost in clubs, South can discard a heart on the third spade. He begins with the safety play in clubs-laying down the King. If the 9, 10, or Queen drops we take the discard on 
Q, then play the clubs to lose only one trick.
If no high card falls under 
K we follow with a club to the Ace. Then if it appears that two club tricks will have to be lost, East having Q 10 9 we finesse the 9 of spades in the hope of establishing two discards.
The unusual feature of the hand is the change back to spades after either defender has dropped the 9 or 10 of clubs on the King.
If …then…